5,000,000 Common Shares of Beneficial Interest

Exhibit 1.1

Execution Version

5,000,000 Common Shares of Beneficial Interest

WASHINGTON REAL ESTATE INVESTMENT TRUST

UNDERWRITING AGREEMENT

April 30, 2009

WACHOVIA CAPITAL MARKETS, LLC

CITIGROUP GLOBAL MARKETS INC.

RAYMOND JAMES & ASSOCIATES, INC.

As Representatives of the Several Underwriters

c/o Wachovia Capital Markets, LLC

375 Park Avenue

New York, NY 10152

Dear Sirs:

1. Introductory. Washington Real Estate Investment Trust, a real estate investment trust organized under the laws of the State of Maryland (“Company”), agrees with the several Underwriters named in Schedule A hereto (the “Underwriters”) to issue and sell to the several Underwriters 5,000,000 of its common shares of beneficial interest (“Firm Securities”) and also proposes to issue and sell to the Underwriters, at the option of the Underwriters, an aggregate of not more than 750,000 additional common shares of beneficial interest (“Optional Securities”) as set forth below. The Firm Securities and the Optional Securities are herein collectively called the “Offered Securities”.

2. Representations and Warranties of the Company. The Company represents and warrants to, and agrees with, the several Underwriters that:

(a) Filing and Effectiveness of Registration Statement. Certain terms are defined at the end of this subsection. The Company has filed with the Commission a registration statement on Form S-3 (No. 333-136921) covering the registration of the Offered Securities under the Act, including a related preliminary prospectus or prospectuses. At any particular time, this initial registration statement, in the form then on file with the Commission, including all material then incorporated by reference therein, all information contained in the registration statement (if any) pursuant to Rule 462(b) and then deemed to be a part of the initial registration statement, and all 430A Information and all 430C Information, that in any case has not then been superseded or modified, shall be referred to as the “Initial Registration Statement”. The Company may also have filed, or may file with the Commission, a Rule 462(b) registration statement covering the registration of Offered Securities. At any particular time, this Rule 462(b) registration statement, in the form then on file with the Commission, including the contents of the Initial Registration Statement incorporated by reference therein and including all 430A Information and all 430C Information, that in any case has not then been superseded or modified, shall be referred to as the “Additional Registration Statement”.

As of the time of execution and delivery of this Agreement, the Initial Registration Statement has been declared effective under the Act and is not proposed to be amended. Any Additional Registration Statement has or will become effective upon filing with the Commission pursuant to Rule 462(b) and is not proposed to be amended. The Offered Securities all have been or will be duly registered under the Act pursuant to the Initial Registration Statement and, if applicable, the Additional Registration Statement.

For purposes of this Agreement:

“1934 Act” means the Securities Exchange Act of 1934, as amended.

“430A Information”, with respect to any registration statement, means information included in a prospectus and retroactively deemed to be a part of such registration statement pursuant to Rule 430A(b).

“430C Information”, with respect to any registration statement, means information included in a prospectus then deemed to be a part of such registration statement pursuant to Rule 430C.

“Act” means the Securities Act of 1933, as amended.

“Applicable Time” means 9:30 a.m. (Eastern time) on the date of this Agreement.

“Closing Date” has the meaning defined in Section 3 hereof.

“Commission” means the Securities and Exchange Commission.

“Effective Date” with respect to the Initial Registration Statement or the Additional Registration Statement (if any) means the date of the Effective Time thereof.

“Effective Time” with respect to the Initial Registration Statement or, if filed prior to the execution and delivery of this Agreement, the Additional Registration Statement means the date and time as of which such Registration Statement was declared effective by the Commission or has become effective upon filing pursuant to Rule 462(c). If an Additional Registration Statement has not been filed prior to the execution and delivery of this Agreement but the Company has advised the Representatives that it proposes to file one, “Effective Time” with respect to such Additional Registration Statement means the date and time as of which such Registration Statement is filed and becomes effective pursuant to Rule 462(b).

“Exchange Act” means the Securities Exchange Act of 1934.

“Final Prospectus” means the Statutory Prospectus that discloses the public offering price, other 430A Information and other final terms of the Offered Securities and otherwise satisfies Section 10(a) of the Act.

“General Use Issuer Free Writing Prospectus” means any Issuer Free Writing Prospectus that is intended for general distribution to prospective investors, as evidenced by its being so specified in Schedule B to this Agreement.

“Issuer Free Writing Prospectus” means any “issuer free writing prospectus,” as defined in Rule 433, relating to the Offered Securities in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g).

“Limited Use Issuer Free Writing Prospectus” means any Issuer Free Writing Prospectus that is not a General Use Issuer Free Writing Prospectus.

A “Registration Statement” at any particular time means the Initial Registration Statement and any Additional Registration Statement as of such time. A “Registration Statement” without reference to a time means such Registration Statement as of its Effective Time. The Initial Registration Statement and the Additional Registration Statement are referred to collectively as the

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“Registration Statements” and individually as a “Registration Statement”. For purposes of the foregoing definitions, 430A Information with respect to a Registration Statement shall be considered to be included in such Registration Statement as of the time specified in Rule 430A.

“Rules and Regulations” means the rules and regulations of the Commission.

“Statutory Prospectus” as of any time means the prospectus included in a Registration Statement immediately prior to that time, including any document incorporated by reference therein and any 430A Information or 430C Information with respect to such Registration Statement. For purposes of the foregoing definition, 430A Information shall be considered to be included in the Statutory Prospectus as of the actual time that form of prospectus is filed with the Commission pursuant to Rule 424(b) or Rule 462(c) and not retroactively.

“Trust Indenture Act” means the Trust Indenture Act of 1939.

Unless otherwise specified, a reference to a “rule” is to the indicated rule under the Act.

(b) Compliance with Securities Act Requirements. (i) On its respective Effective Date, on the date of this Agreement and on each Closing Date, each of the Initial Registration Statement and the Additional Registration Statement (if any) conformed and will conform in all material respects to the requirements of the Act and the Rules and Regulations and did not and will not include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading, and (ii) on the date of this Agreement, and at the time of filing of the Final Prospectus pursuant to Rule 424(b) or (if no such filing is required) at the Effective Date of the Additional Registration Statement in which the Final Prospectus is included, the Final Prospectus will conform in all material respects to the requirements of the Act and the Rules and Regulations and will not include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading. The preceding sentence does not apply to statements in or omissions from any such document based upon written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information is that described as such in Section 8(b) hereof.

(c) Incorporated Documents. The documents incorporated or deemed to be incorporated by reference in the Registration Statement and the Prospectus, at the time they were or hereafter are filed with the Commission, complied and will comply in all material respects with the requirements of the 1934 Act and the Rules and Regulations.

(d) Ineligible Issuer Status. (i) At the time of initial filing of the Initial Registration Statement and (ii) at the date of this Agreement, the Company was not and is not an “ineligible issuer,” as defined in Rule 405, including (x) the Company or any other subsidiary in the preceding three years not having been convicted of a felony or misdemeanor or having been made the subject of a judicial or administrative decree or order as described in Rule 405 and (y) the Company in the preceding three years not having been the subject of a bankruptcy petition or insolvency or similar proceeding, not having had a registration statement be the subject of a proceeding under Section 8 of the Act and not being the subject of a proceeding under Section 8A of the Act in connection with the offering of the Offered Securities, all as described in Rule 405. At the time the Company or any person acting on its behalf (within the meaning, for this sentence only, of Rule 163(c)) made any offer in reliance on the exemption of Rule 163, the Company was a “well-known seasoned issuer” as defined in Rule 405, including not having been an “ineligible issuer” as defined in Rule 405.

(e) General Disclosure Package. As of the Applicable Time, neither (i) the General Use Issuer Free Writing Prospectus(es) issued at or prior to the Applicable Time, the preliminary prospectus, dated April 29, 2009 (which is the most recent Statutory Prospectus distributed to

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investors generally) and the other information, if any, stated in Schedule B to this Agreement to be included in the General Disclosure Package, all considered together (collectively, the “General Disclosure Package”), nor (ii) any individual Limited Use Issuer Free Writing Prospectus, when considered together with the General Disclosure Package, included any untrue statement of a material fact or omitted to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The preceding sentence does not apply to statements in or omissions from any Statutory Prospectus or any Issuer Free Writing Prospectus in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in Section 8(b) hereof.

(f) Information 8-K. As of the time it was furnished to the Commission and as of the Applicable Time, the Information 8-K (as defined below), did not, and as of the First Closing Date and as of the Optional Closing Date, as the case may be, will not, contain any untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. As of the time the Information 8-K was furnished to the Commission, it conformed in all material respects with the requirements of the 1934 Act and the Rules and Regulations. The “Information 8-K” means the Company’s Current Report on Form 8-K furnished to the Commission on April 28, 2009 and Exhibit 99.1 thereto.

(g) Issuer Free Writing Prospectuses. Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the completion of the public offer and sale of the Offered Securities or until any earlier date that the Company notified or notifies the Representatives as described in the next sentence, did not, does not and will not include any information that conflicted, conflicts or will conflict with the information then contained in the Registration Statement. If at any time following issuance of an Issuer Free Writing Prospectus there occurred or occurs an event or development as a result of which such Issuer Free Writing Prospectus conflicted or would conflict with the information then contained in the Registration Statement or included or would include an untrue statement of a material fact or omitted or would omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances prevailing at that subsequent time, not misleading, (i) the Company has promptly notified or will promptly notify the Representatives and (ii) the Company has promptly amended or will promptly amend or supplement such Issuer Free Writing Prospectus to eliminate or correct such conflict, untrue statement or omission.

(h) Good Standing of the Company. The Company has been duly organized and is validly existing as a real estate investment trust of unlimited duration with transferable shares of beneficial interest in good standing under the laws of the State of Maryland, with full power and authority to own, lease and operate its properties and to conduct its business as described in the General Disclosure Package and to enter into this Agreement and to consummate the transactions contemplated hereby. The Company is duly qualified to transact business and is in good standing in each jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure to so qualify or be in good standing would not result in a material adverse change in the condition, financial or otherwise, or the results of operations, business, properties or prospects of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business (a “Material Adverse Effect”).

(i) Subsidiaries. Each “significant subsidiary” of the Company (as such term is defined in Rule 1-02 of Regulation S-X promulgated under the 1933 Act) (each, a “Subsidiary” and, collectively, the “Subsidiaries”) (which term includes corporations, limited and general partnerships, limited liability companies, joint ventures and other entities, and includes direct and

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indirect subsidiaries), if any, has been duly organized and is validly existing as a corporation, limited liability company or partnership, as the case may be, in good standing under the laws of the jurisdiction of its incorporation or organization, has power and authority to own, lease and operate its properties and to conduct its business as described in the General Disclosure Package and is duly qualified to transact business and is in good standing in each jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure to so qualify or be in good standing would not result in a Material Adverse Effect. Except as otherwise stated in the General Disclosure Package, all of the issued and outstanding capital stock or other ownership interests of each Subsidiary have been duly authorized and validly issued, are fully paid and non-assessable and are owned by the Company, directly or through subsidiaries, free and clear of any security interest, mortgage, pledge, lien, encumbrance, claim or equity; and none of the outstanding shares of capital stock of any Subsidiary was issued in violation of preemptive or other similar rights of any security holder of such Subsidiary.

(j) Offered Securities. The Offered Securities and all other outstanding shares of beneficial interest of the Company have been duly authorized; the authorized equity capitalization of the Company is as set forth in the General Disclosure Package; all outstanding shares of beneficial interest of the Company are, and, when the Offered Securities have been delivered and paid for in accordance with this Agreement on each Closing Date, such Offered Securities will have been, validly issued, fully paid and nonassessable, will be consistent with the information in the General Disclosure Package and will conform to the description thereof contained in the Final Prospectus; the shareholders of the Company have no preemptive rights with respect to the Offered Securities; and none of the outstanding shares of beneficial interest of the Company have been issued in violation of any preemptive or similar rights of any security holder.

(k) Finder’s Fee. Except as disclosed in the General Disclosure Package, there are no contracts, agreements or understandings between the Company and any person that would give rise to a valid claim against the Company or any Underwriter for a brokerage commission, finder’s fee or other like payment in connection with this offering.

(l) Registration Rights. Except as disclosed in the General Disclosure Package, there are no contracts, agreements or understandings between the Company and any person granting such person the right to require the Company to file a registration statement under the Act with respect to any securities of the Company owned or to be owned by such person or to require the Company to include such securities in the securities registered pursuant to a Registration Statement or in any securities being registered pursuant to any other registration statement filed by the Company under the Act (collectively, “registration rights”), and any person to whom the Company has granted registration rights has agreed not to exercise such rights until after the expiration of the Lock-Up Period referred to in Section 5 hereof.

(m) Listing. As of the First Closing Date, the Offered Securities will have been approved for listing on the New York Stock Exchange, subject to notice of issuance.

(n) Absence of Further Requirements. No consent, approval, authorization, or order of, or filing or registration with, any person (including any governmental agency or body or any court) is required for the consummation by the Company of the transactions contemplated by this Agreement in connection with the offering, issuance and sale of the Offered Securities by the Company, except such as have been obtained, or made and such as may be required under state securities laws.

(o) Title to Property. The Company and its subsidiaries have good and marketable title to, or valid and enforceable leasehold estates in, all items of real and personal property referred to in the General Disclosure Package as owned or leased by them, in each case free and clear of all liens,

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encumbrances, claims, security interests and defects, other than those referred to in the General Disclosure Package or that are not material in amount. The Company has no reason to believe that the lessee under any lease (excluding leases for which rent payments due for the remainder of such lease are less than $1,000,000) calling for annual lease payments in excess of $1,000,000 is not financially capable of performing its obligations thereunder.

(p) Absence of Defaults and Conflicts Resulting from Transaction. The execution, delivery and performance of this Agreement, and the issuance and sale of the Offered Securities will not result in a breach or violation of any of the terms and provisions of, or constitute a default or a Debt Repayment Triggering Event (as defined below) under, or result in the imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its subsidiaries pursuant to, the declaration of trust, partnership agreement, charter or by-laws or other organizational documents of the Company or any of its subsidiaries, any statute, rule, regulation or order of any governmental agency or body or any court, domestic or foreign, having jurisdiction over the Company or any of its subsidiaries or any of their properties, or any agreement or instrument to which the Company or any of its subsidiaries is a party or by which the Company or any of its subsidiaries is bound or to which any of the properties of the Company or any of its subsidiaries is subject; a “Debt Repayment Triggering Event” means any event or condition that gives, or with the giving of notice or lapse of time would give, the holder of any note, debenture, or other evidence of indebtedness (or any person acting on such holder’s behalf) the right to require the repurchase, redemption or repayment of all or a portion of such indebtedness by the Company or any of its subsidiaries.

(q) No Violation or Default. Neither the Company nor any of its subsidiaries is (i) in violation of its respective declaration of trust, partnership agreement, charter or by-laws or other organizational document or (ii) in default (or with the giving of notice or lapse of time would be in default) under any existing obligation, agreement, covenant or condition contained in any indenture, loan agreement, mortgage, lease or other agreement or instrument to which any of them is a party or by which any of them is bound or to which any of the properties of any of them is subject or (iii) in violation of any law or statute or any judgment, order, rule or regulation of any court or arbitrator or governmental or regulatory authority, except, in the case of clauses (ii) and (iii) above, for any such default or violation that would not, individually or in the aggregate, have a Material Adverse Effect.

(r) Authorization of Agreement. This Agreement has been duly authorized, executed and delivered by the Company.

(s) Possession of Licenses and Permits. The Company and its subsidiaries possess, and are in compliance with the terms of, all certificates, authorizations, franchises, licenses and permits (“Licenses”) necessary or material to the conduct of the business now conducted or proposed in the General Disclosure Package to be conducted by them and have not received any notice of proceedings relating to the revocation or modification of any Licenses that, if determined adversely to the Company or any of its subsidiaries, would individually or in the aggregate have a Material Adverse Effect.

(t) Absence of Labor Dispute. No labor dispute with the employees of the Company or any of its subsidiaries exists or, to the knowledge of the Company, is imminent, and the Company is not aware of any existing or imminent labor disturbance by the employees of any of its or its subsidiaries’ principal suppliers, contractors or customers, that, in any such case, could have a Material Adverse Effect.

(u) Possession of Intellectual Property. The Company and its subsidiaries own, possess or can acquire on reasonable terms, adequate trademarks, trade names and other rights to inventions, know-how, patents, copyrights, confidential information and other intellectual property

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(collectively, “intellectual property rights”) necessary to conduct the business now operated by them, or presently employed by them, and have not received any notice of infringement of or conflict with asserted rights of others with respect to any intellectual property rights that, if determined adversely to the Company or any of its subsidiaries, would individually or in the aggregate have a Material Adverse Effect.

(v) Environmental Laws. Except as disclosed in the General Disclosure Package, neither the Company nor any of its subsidiaries is in violation of any statute, any rule, regulation, decision or order of any governmental agency or body or any court, domestic or foreign, relating to the use, disposal or release of hazardous or toxic substances or relating to the protection or restoration of the environment or human exposure to hazardous or toxic substances (collectively, “environmental laws”), owns or operates any real property contaminated with any substance that is subject to any environmental laws, is liable for any off-site disposal or contamination pursuant to any environmental laws, or is subject to any claim relating to any environmental laws, which violation, contamination, liability or claim would individually or in the aggregate have a Material Adverse Effect; and the Company is not aware of any pending investigation which might lead to such a claim.

(w) Accurate Disclosure. The statements in the General Disclosure Package and the Final Prospectus under the heading “Description of Shares” and under the heading “Risk Factors” in the Company’s 2008 Annual Report on Form 10-K and the statements in the Registration Statement under Item 15 insofar as such statements summarize legal matters, agreements, documents or proceedings discussed therein, are accurate and fair summaries of such legal matters, agreements, documents or proceedings and present the information required to be shown in all material respects.

(x) REIT Status. With respect to all tax periods regarding which the Internal Revenue Service is or will be entitled to assert any claim, the Company has met the requirements for qualification as a real estate investment trust under Sections 856 through 860 of the Internal Revenue Code, as amended, and the Company’s present and contemplated operations, assets and income continue to meet such requirements.

(y) Insurance. The Company and its subsidiaries are insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as are prudent and customary in the businesses in which they are engaged; the Company has no reason to believe that it or any of its subsidiaries will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage from similar insurers as may be necessary to continue its business at a cost that would not have a Material Adverse Effect; and the Company and each of its subsidiaries has obtained title insurance on all of the properties owned by each of them in an amount at least equal to the original purchase price to acquire land and improvements and such title insurance is in full force and effect.

(z) Taxes. The Company has filed all federal, state and foreign income tax returns that have been required to be filed and has paid all taxes indicated by said returns and all assessments received by it to the extent that such taxes have become due.

(aa) Statistical and Market-Related Data. Any third-party statistical and market-related data included or incorporated by reference in a Registration Statement or a Statutory Prospectus are based on or derived from sources that the Company believes to be reliable and accurate.

(bb) Compliance with the Sarbanes-Oxley Act. Except as set forth in the General Disclosure Package, (i) the Company, its subsidiaries and the Company’s Board of Trustees (the “Board”) are in compliance in all material respects with Sarbanes-Oxley (as defined below) and all

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applicable Exchange Rules (as defined below); (ii) the Company has adopted and has or, upon consummation of the offering of the Offered Securities, will have disclosed corporate governance guidelines; (iii) the Company maintains a system of internal controls, including, but not limited to, disclosure controls and procedures, internal control over financial reporting, an internal audit function and legal and regulatory compliance controls (collectively, “Internal Controls”), which complies with the Securities Laws (as defined below) and is or, upon consummation of the offering of the Offered Securities will be overseen by the Audit Committee (the “Audit Committee”) of the Board in accordance with Exchange Rules; (iv) since the disclosure in May 9, 2007 of a weakness in disclosure controls, the Company has not publicly disclosed or reported to the Audit Committee or the Board, and within the next 90 days the Company does not reasonably expect to publicly disclose or report to the Audit Committee or the Board, a significant deficiency, material weakness, change in Internal Controls or fraud involving management or other employees who have a significant role in Internal Controls (each, an “Internal Control Event”), any violation of, or failure to comply with, the Securities Laws, or any matter which, if determined adversely, would have a Material Adverse Effect; and (v) “Securities Laws” means, collectively, the Sarbanes-Oxley Act of 2002 (“Sarbanes-Oxley”), the Act, the Exchange Act, the Rules and Regulations, the auditing principles, rules, standards and practices applicable to auditors of “issuers” (as defined in Sarbanes-Oxley) promulgated or approved by the Public Company Accounting Oversight Board (“PCAOB Rules” and “PCAOB,” respectively), and, as applicable, the rules of the New York Stock Exchange (“Exchange Rules”).

(cc) Litigation. Except as disclosed in the General Disclosure Package, there are no pending actions, suits or proceedings (including any inquiries or investigations by any court or governmental agency or body, domestic or foreign) against or affecting the Company, any of its subsidiaries or any of their respective properties that, if determined adversely to the Company or any of its subsidiaries, would individually or in the aggregate have a Material Adverse Effect, or would materially and adversely affect the ability of the Company to perform its obligations under this Agreement, or which are otherwise material in the context of the sale of the Offered Securities; and no such actions, suits or proceedings (including any inquiries or investigations by any court or governmental agency or body, domestic or foreign) are threatened or, to the Company’s knowledge, contemplated.

(dd) Financial Statements and Independent Accountants. The financial statements included in each Registration Statement and the General Disclosure Package present fairly the financial position of the Company and its consolidated subsidiaries as of the dates shown and their results of operations and cash flows for the periods shown, and such financial statements have been prepared in conformity with the generally accepted accounting principles in the United States (“GAAP”) applied on a consistent basis and the schedules included in each Registration Statement present fairly the information required to be stated therein; and the assumptions used in preparing the pro forma financial statements included in each Registration Statement and the General Disclosure Package provide a reasonable basis for presenting the significant effects directly attributable to the transactions or events described therein, the related pro forma adjustments give appropriate effect to those assumptions, and the pro forma columns therein reflect the proper application of those adjustments to the corresponding historical financial statement amounts. The supporting schedules, if any, included in the Registration Statement and the General Disclosure Package present fairly in accordance with GAAP the information required to be stated therein. The selected financial data included in the General Disclosure Package present fairly the information shown therein and have been compiled on a basis consistent with that of the audited financial statements included in the Registration Statement and the General Disclosure Package. The historical summaries of revenue and certain operating expenses included in the Registration Statement and the General Disclosure Package present fairly the revenue and those operating expenses included in such summaries of the properties related thereto for the periods specified in conformity with GAAP. The accountants who certified the financial statements and any supporting schedules thereto included in each Registration Statement and the General Disclosure Package are independent public accountants within the meaning of the Securities Laws.

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(ee) Accuracy of Exhibits. There are no contracts or documents that are required to be described in the Registration Statement, the Prospectus or the documents incorporated by reference therein or to be filed as exhibits thereto that have not been so described and filed as required.

(ff) No Material Adverse Change in Business. Except as disclosed in the General Disclosure Package, since the end of the period covered by the latest audited financial statements included in the General Disclosure Package (i) there has been no change, nor any development or event involving a prospective change, in the condition (financial or otherwise), results of operations, business, properties or prospects of the Company and its subsidiaries, taken as a whole that is material and adverse, (ii) except as disclosed in or contemplated by the General Disclosure Package, there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock and (iii) except as disclosed in or contemplated by the General Disclosure Package, there has been no material adverse change in the capital stock, short-term indebtedness, long-term indebtedness, net current assets or net assets of the Company and its subsidiaries.

(gg) Investment Company Act. The Company is not and, after giving effect to the offering and sale of the Offered Securities and the application of the proceeds thereof as described in the General Disclosure Package, will not be an “investment company” as defined in the Investment Company Act of 1940 (the “Investment Company Act”).

(hh) Ratings. No “nationally recognized statistical rating organization” as such term is defined for purposes of Rule 436(g)(2) (i) has imposed (or has informed the Company that it is considering imposing) any condition (financial or otherwise) on the Company’s retaining any rating assigned to the Company or any securities of the Company or (ii) has indicated to the Company that it is considering any of the actions described in Section 7(c)(ii) hereof.

3. Purchase, Sale and Delivery of Offered Securities. On the basis of the representations, warranties and agreements and subject to the terms and conditions set forth herein, the Company agrees to sell to the several Underwriters, and each of the Underwriters agrees, severally and not jointly, to purchase from the Company, at a purchase price of $20.51 per share, the respective number of Firm Securities set forth opposite the names of the Underwriters set forth in Schedule A hereto.

The Company will deliver the Firm Securities to or as instructed by the Representatives for the accounts of the several Underwriters in a form reasonably acceptable to the Representatives against payment of the purchase price by the Underwriters in Federal (same day) funds by wire transfer to an account at a bank acceptable to the Representatives drawn to the order of Washington Real Estate Investment Trust at the office of Vinson & Elkins LLP, 1455 Pennsylvania Avenue, Washington, DC, 20004, at 10:00 A.M., New York time, on May 5, 2009, or at such other time not later than seven full business days thereafter as the Representatives and the Company determine, such time being herein referred to as the “First Closing Date”. For purposes of Rule 15c6-1 under the Securities Exchange Act of 1934, the First Closing Date (if later than the otherwise applicable settlement date) shall be the settlement date for payment of funds and delivery of securities for all the Offered Securities sold pursuant to the offering. The Firm Securities so to be delivered or evidence of their issuance will be made available for checking at the above office of Vinson & Elkins LLP at least 24 hours prior to the First Closing Date.

In addition, upon written notice from the Representatives given to the Company from time to time not more than 30 days subsequent to the date of the Final Prospectus, the Underwriters may purchase all or less than all of the Optional Securities at the purchase price per Security to be paid for the Firm Securities

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less an amount equal to any dividend paid by the Company and payable on any Firm Securities and not payable on such Optional Securities. The Company agrees to sell to the Underwriters the number of Optional Securities specified in such notice and the Underwriters agree, severally and not jointly, to purchase such Optional Securities. Such Optional Securities shall be purchased for the account of each Underwriter in the same proportion as the number of Firm Securities set forth opposite such Underwriter’s name bears to the total number of Firm Securities (subject to adjustment by the Representatives to eliminate fractions) and may be purchased by the Underwriters only for the purpose of covering over-allotments made in connection with the sale of the Firm Securities. No Optional Securities shall be sold or delivered unless the Firm Securities previously have been, or simultaneously are, sold and delivered. The right to purchase the Optional Securities or any portion thereof may be exercised from time to time and to the extent not previously exercised may be surrendered and terminated at any time upon notice by the Representatives to the Company.

Each time for the delivery of and payment for the Optional Securities, being herein referred to as an “Optional Closing Date”, which may be the First Closing Date (the First Closing Date and each Optional Closing Date, if any, being sometimes referred to as a “Closing Date”), shall be determined by the Representatives but shall be not later than five full business days after written notice of election to purchase Optional Securities is given. The Company will deliver the Optional Securities being purchased on each Optional Closing Date to or as instructed by the Representatives for the accounts of the several Underwriters in a form reasonably acceptable to the Representatives against payment of the purchase price therefor in Federal (same day) funds by wire transfer to an account at a bank acceptable to the Representatives drawn to the order of Washington Real Estate Investment Trust, at the above office of Vinson & Elkins LLP. The Optional Securities being purchased on each Optional Closing Date or evidence of their issuance will be made available for checking at the above office of Vinson & Elkins LLP at a reasonable time in advance of such Optional Closing Date.

4. Offering by Underwriters. It is understood that the several Underwriters propose to offer the Offered Securities for sale to the public as set forth in the Final Prospectus.

5. Certain Agreements of the Company. The Company agrees with the several Underwriters that:

(a) Additional Filings. Unless filed pursuant to Rule 462(c) as part of the Additional Registration Statement in accordance with the next sentence, the Company will file the Final Prospectus, in a form approved by the Representatives, with the Commission pursuant to and in accordance with subparagraph (1) (or, if applicable and if consented to by the Representatives, subparagraph (4)) of Rule 424(b) not later than the earlier of (A) the second business day following the execution and delivery of this Agreement or (B) the fifteenth business day after the Effective Date of the Initial Registration Statement. The Company will advise the Representatives promptly of any such filing pursuant to Rule 424(b) and provide satisfactory evidence to the Representatives of such timely filing. If an Additional Registration Statement is necessary to register a portion of the Offered Securities under the Act but the Effective Time thereof has not occurred as of the execution and delivery of this Agreement, the Company will file the additional registration statement or, if filed, will file a post-effective amendment thereto with the Commission pursuant to and in accordance with Rule 462(b) on or prior to 10:00 P.M., New York time, on the date of this Agreement or, if earlier, on or prior to the time the Final Prospectus is finalized and distributed to any Underwriter, or will make such filing at such later date as shall have been consented to by the Representatives.

(b) Filing of Amendments; Response to Commission Requests. The Company will promptly advise the Representatives of any proposal to amend or supplement at any time the Initial Registration Statement, any Additional Registration Statement or any Statutory Prospectus and will not effect such amendment or supplementation without the Representatives’ consent; and the Company will also advise the Representatives promptly of (i) the effectiveness of any Additional Registration Statement (if its Effective Time is subsequent to the execution and delivery of this

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Agreement), (ii) any amendment or supplementation of a Registration Statement or any Statutory Prospectus, (iii) any request by the Commission or its staff for any amendment to any Registration Statement, for any supplement to any Statutory Prospectus or for any additional information, (iv) the institution by the Commission of any stop order proceedings in respect of a Registration Statement or the threatening of any proceeding for that purpose, and (v) the receipt by the Company of any notification with respect to the suspension of the qualification of the Offered Securities in any jurisdiction or the institution or threatening of any proceedings for such purpose. The Company will use its best efforts to prevent the issuance of any such stop order or the suspension of any such qualification and, if issued, to obtain as soon as possible the withdrawal thereof.

(c) Continued Compliance with Securities Laws. If, at any time when a prospectus relating to the Offered Securities is (or but for the exemption in Rule 172 would be) required to be delivered under the Act by any Underwriter or dealer, any event occurs as a result of which the Final Prospectus as then amended or supplemented would include an untrue statement of a material fact or omit to state any material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, or if it is necessary at any time to amend the Registration Statement or supplement the Final Prospectus to comply with the Act, the Company will promptly notify the Representatives of such event and will promptly prepare and file with the Commission and furnish, at its own expense, to the Underwriters and the dealers and any other dealers upon request of the Representatives, an amendment or supplement which will correct such statement or omission or an amendment which will effect such compliance. Neither the Representatives’ consent to, nor the Underwriters’ delivery of, any such amendment or supplement shall constitute a waiver of any of the conditions set forth in Section 7 hereof.

(d) Rule 158. As soon as practicable, but not later than the Availability Date (as defined below), the Company will make generally available to its securityholders an earnings statement covering a period of at least 12 months beginning after the Effective Date of the Initial Registration Statement (or, if later, the Effective Date of the Additional Registration Statement) which will satisfy the provisions of Section 11(a) of the Act and Rule 158 under the Act. For the purpose of the preceding sentence, “Availability Date” means the day after the end of the fourth fiscal quarter following the fiscal quarter that includes such Effective Date on which the Company is required to file its Form 10-Q for such fiscal quarter except that, if such fourth fiscal quarter is the last quarter of the Company’s fiscal year, “Availability Date” means the day after the end of such fourth fiscal quarter on which the Company is required to file its Form 10-K.

(e) Furnishing of Prospectuses. The Company will furnish to the Representatives copies of each Registration Statement (two of which will be signed and will include all exhibits), each related Statutory Prospectus, and, so long as a prospectus relating to the Offered Securities is (or but for the exemption in Rule 172 would be) required to be delivered under the Act, the Final Prospectus and all amendments and supplements to such documents, in each case in such quantities as the Representatives request. The Final Prospectus shall be so furnished on or prior to 3:00 P.M., New York time, on the business day following the execution and delivery of this Agreement. All other documents shall be so furnished as soon as available. The Company will pay the expenses of printing and distributing to the Underwriters all such documents.

(f) Blue Sky Qualifications. The Company will arrange for the qualification of the Offered Securities for sale under the laws of such jurisdictions as the Representatives designate and will continue such qualifications in effect so long as required for the distribution.

(g) Reporting Requirements. During the period of five years hereafter, the Company will furnish to the Representatives and, upon request, to each of the other Underwriters, as soon as practicable after the end of each fiscal year, a copy of its annual report to shareholders for such year; and the Company will furnish to the Representatives (i) as soon as available, a copy of each

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report and any definitive proxy statement of the Company filed with the Commission under the Exchange Act or mailed to stockholders, and (ii) from time to time, such other information concerning the Company as the Representatives may reasonably request. However, so long as the Company is subject to the reporting requirements of either Section 13 or Section 15(d) of the Exchange Act and is timely filing reports with the Commission on its Electronic Data Gathering, Analysis and Retrieval system (“EDGAR”), it is not required to furnish such reports or statements to the Underwriters.

(h) Payment of Expenses. The Company will pay all expenses incident to the performance of its obligations under this Agreement, including but not limited to any filing fees and other expenses (including fees and disbursements of counsel to the Underwriters) incurred in connection with qualification of the Offered Securities for sale under the laws of such jurisdictions as the Representatives designate and the preparation and printing of memoranda relating thereto, costs and expenses relating to investor presentations or any “road show” in connection with the offering and sale of the Offered Securities including, without limitation, any travel expenses of the Company’s officers and employees and any other expenses of the Company including the chartering of airplanes, fees and expenses incident to listing the Offered Securities on the New York Stock Exchange, fees and expenses in connection with the registration of the Offered Securities under the Exchange Act, and expenses incurred in distributing preliminary prospectuses and the Final Prospectus (including any amendments and supplements thereto) to the Underwriters and for expenses incurred for preparing, printing and distributing any Issuer Free Writing Prospectuses to investors or prospective investors.

(i) Use of Proceeds. The Company will use the net proceeds received in connection with this offering in the manner described in the “Use of Proceeds” section of the General Disclosure Package and, except as disclosed in the General Disclosure Package, the Company does not intend to use any of the proceeds from the sale of the Offered Securities hereunder to repay any outstanding debt owed to any affiliate of any Underwriter.

(j) Absence of Manipulation. The Company will not take, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, stabilization or manipulation of the price of any securities of the Company to facilitate the sale or resale of the Offered Securities.

(k) Restriction on Sale of Securities. For the period specified below (the “Lock-Up Period”), the Company will not, directly or indirectly, take any of the following actions with respect to its common shares of beneficial interest or any securities convertible into or exchangeable or exercisable for any of its common shares of beneficial interest (“Lock-Up Securities”): (i) offer, sell, issue, contract to sell, pledge or otherwise dispose of Lock-Up Securities, (ii) offer, sell, issue, contract to sell, contract to purchase or grant any option, right or warrant to purchase Lock-Up Securities, (iii) enter into any swap, hedge or any other agreement that transfers, in whole or in part, the economic consequences of ownership of Lock-Up Securities, (iv) establish or increase a put equivalent position or liquidate or decrease a call equivalent position in Lock-Up Securities within the meaning of Section 16 of the Exchange Act or (v) file with the Commission a registration statement under the Act relating to Lock-Up Securities, or publicly disclose the intention to take any such action, without the prior written consent of the Representatives except grants of employee stock options pursuant to the terms of a plan in effect on the date hereof, issuances of Lock-Up Securities pursuant to the exercise of such options or the exercise of any other employee stock options outstanding on the date hereof or issuances of Lock-Up Securities pursuant to the Company’s dividend reinvestment plan. The initial Lock-Up Period will commence on the date hereof and continue for 30 days after the date of the commencement of the public offering of the Offered Securities or such earlier date that the Representatives consent to in writing; provided, however, that if (1) during the last 17 days of the initial Lock-Up Period, the Company releases earnings results or material news or a material event relating to the Company

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occurs or (2) prior to the expiration of the initial Lock-Up Period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the initial Lock-Up Period, then in each case the Lock-Up Period will be extended until the expiration of the 18-day period beginning on the date of release of the earnings results or the occurrence of the materials news or material event, as applicable, unless the Representatives waive, in writing, such extension. The Company will provide the Representatives with notice of any announcement described in clause (2) of the preceding sentence that gives rise to an extension of the Lock-Up Period.

(l) REIT Status. The Company will use its best efforts to continue to meet the requirements to qualify as a “real estate investment trust” under the Internal Revenue Code of 1986, as amended.

6. Issuer Free Writing Prospectuses. The Company represents and agrees that, unless it obtains the prior consent of the Representatives, and each Underwriter represents and agrees that, unless it obtains the prior consent of the Company and Representatives, it has not made and will not make any offer relating to the Offered Securities that would constitute an Issuer Free Writing Prospectus, or that would otherwise constitute a “free writing prospectus,” as defined in Rule 405, required to be filed with the Commission. Any such free writing prospectus consented to by the Company and the Representatives is hereinafter referred to as a “Permitted Free Writing Prospectus.” The Company represents that it has treated and agrees that it will treat each Permitted Free Writing Prospectus as an “issuer free writing prospectus,” as defined in Rule 433, and has complied and will comply with the requirements of Rules 164 and 433 applicable to any Permitted Free Writing Prospectus, including timely Commission filing where required, legending and record keeping.

7. Conditions of the Obligations of the Underwriters. The obligations of the several Underwriters to purchase and pay for the Firm Securities on the First Closing Date and the Optional Securities to be purchased on each Optional Closing Date will be subject to the accuracy of the representations and warranties of the Company herein (as though made on such Closing Date), to the accuracy of the statements of Company officers made pursuant to the provisions hereof, to the performance by the Company of its obligations hereunder and to the following additional conditions precedent:

(a) Accountants’ Comfort Letter. The Representatives shall have received letters, dated, respectively, the date hereof and each Closing Date, of Ernst & Young LLP confirming that Ernst & Young LLP is a registered public accounting firm and independent public accountants within the meaning of the Securities Laws and substantially in the form of Schedule C hereto (except that, in any letter dated a Closing Date, the specified date referred to in Schedule C hereto shall be a date no more than three days prior to such Closing Date).

(b) Effectiveness of Registration Statement. If the Effective Time of the Additional Registration Statement (if any) is not prior to the execution and delivery of this Agreement, such Effective Time shall have occurred not later than 10:00 P.M., New York time, on the date of this Agreement or, if earlier, the time the Final Prospectus is finalized and distributed to any Underwriter, or shall have occurred at such later time as shall have been consented to by the Representatives. The Final Prospectus shall have been filed with the Commission in accordance with the Rules and Regulations and Section 5(a) hereof. Prior to such Closing Date, no stop order suspending the effectiveness of a Registration Statement shall have been issued and no proceedings for that purpose shall have been instituted or, to the knowledge of the Company or the Underwriters, shall be contemplated by the Commission.

(c) No Material Adverse Change. Subsequent to the execution and delivery of this Agreement, there shall not have occurred (i) any change, or any development or event involving a prospective change, in the condition (financial or otherwise), results of operations, business, properties or prospects of the Company and its subsidiaries taken as a whole which, in the

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judgment of the Representatives, is material and adverse and makes it impractical or inadvisable to market the Offered Securities; (ii) any downgrading in the rating of any debt securities of the Company by any “nationally recognized statistical rating organization” (as defined for purposes of Rule 436(g)), or any public announcement that any such organization has under surveillance or review its rating of any debt securities of the Company (other than an announcement with positive implications of a possible upgrading, and no implication of a possible downgrading, of such rating); (iii) any change in U.S. or international financial, political or economic conditions or currency exchange rates or exchange controls the effect of which is such as to make it, in the judgment of the Representatives, impractical to market or to enforce contracts for the sale of the Offered Securities, whether in the primary market or in respect of dealings in the secondary market; (iv) any suspension or material limitation of trading in securities generally on the New York Stock Exchange or any setting of minimum or maximum prices for trading on such exchange; (v) or any suspension of trading of any securities of the Company on any exchange or in the over-the-counter market; (vi) any banking moratorium declared by any U.S. federal or New York authorities; (vii) any major disruption of settlements of securities, payment, or clearance services in the United States or any other country where such securities are listed; or (viii) any attack on, outbreak or escalation of hostilities or act of terrorism involving the United States, any declaration of war by Congress or any other national or international calamity or emergency if, in the judgment of the Representatives, the effect of any such attack, outbreak, escalation, act, declaration, calamity or emergency is such as to make it impractical or inadvisable to market the Offered Securities or to enforce contracts for the sale of the Offered Securities.

(d) Opinion of Counsel for Company. The Representatives shall have received an opinion, dated such Closing Date, of Arent Fox LLP, counsel for the Company, to the effect that:

(i) Good Standing of the Company. The Company has been duly organized and is validly existing as a real estate investment trust in good standing under the laws of the State of Maryland, with trust power and authority to own, lease and operate its properties and conduct its business as described in the General Disclosure Package and to enter into this Agreement and to consummate the transactions contemplated in the General Disclosure Package; and at the time of the filing of the Registration Statement and the time of the filing of any post-effective amendment to the Registration Statement (including the filing of the Annual Report on Form 10-K), the conditions for use of a registration statement on Form S-3 were satisfied. The Company is duly qualified to transact business in each jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure to so qualify would not result in a Material Adverse Effect.

(ii) Subsidiaries. Each Subsidiary has been duly organized and is validly existing as a corporation or partnership, as the case may be, in good standing under the laws of the jurisdiction of its organization and has corporate or partnership power and authority, as the case may be, to own, lease and operate its properties and to conduct its business as described in the General Disclosure Package. Except as otherwise stated in the General Disclosure Package, all of the issued and outstanding capital stock or other ownership interests of each Subsidiary have been duly authorized and are validly issued, fully paid and non-assessable and, to the knowledge of such counsel, are owned by the Company, directly or through subsidiaries, and are owned free and clear of any security interest, mortgage, pledge, lien, encumbrance, claim or equity. To such counsel’s knowledge, none of the outstanding shares of capital stock of any Subsidiary was issued in violation of preemptive or other similar rights of any security holder of such Subsidiary. Each Subsidiary of the Company is duly qualified as a foreign corporation or partnership to transact business and is in good standing in each jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure to so qualify or be in good standing would not result in a Material Adverse Effect.

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(iii) Offered Securities; Capitalization. The Offered Securities delivered on such Closing Date and all other outstanding shares of beneficial interest of the Company have been duly authorized and validly issued, are fully paid and nonassessable, are consistent with the information in the General Disclosure Package and conform to the description thereof contained in the Final Prospectus; the authorized equity capitalization of the Company is as set forth in the General Disclosure Package; the shareholders of the Company have no preemptive rights with respect to the Offered Securities; and none of the outstanding shares of beneficial interest of the Company have been issued in violation of any preemptive or similar rights of any security holder arising by operation of law or, to such counsel’s knowledge, otherwise; the form of certificate used to evidence the Offered Securities is in due and proper form and complies with all applicable statutory requirements.

(iv) Registration Rights. To the knowledge of such counsel, there are no contracts, agreements or understandings between the Company and any person granting such person registration rights.

(v) Investment Company Act. The Company is not and, after giving effect to the offering and sale of the Offered Securities and the application of the proceeds thereof as described in the General Disclosure Package, will not be an “investment company” as defined in the Investment Company Act.

(vi) Absence of Further Requirements. No consent, approval, authorization or order of, or filing with, any person (including any governmental agency or body or any court) is required for the consummation by the Company of the transactions contemplated by this Agreement in connection with the offering, issuance and sale of the Offered Securities by the Company, except such as have been obtained or made and such as may be required under state securities laws.

(vii) Absence of Defaults and Conflicts Resulting from Transaction. The execution, delivery and performance of this Agreement and the issuance and sale of the Offered Securities have been duly authorized by the Company and will not result in a breach or violation of any of the terms and provisions of, or constitute a default under, or result in the imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its subsidiaries pursuant to the declaration of trust, partnership agreement, charter or by-laws or other organizational documents of the Company or of its subsidiaries, any statute, rule, regulation or order of any governmental agency or body or any court having jurisdiction over the Company or any of its subsidiaries or any of their properties or any contract, indenture, mortgage, loan agreement, note, lease or other agreement or instrument that is described or referred to in a Registration Statement or the General Disclosure Package or filed or incorporated by reference as an exhibit to a Registration Statement.

(viii) Compliance with Registration Requirements; Effectiveness. The Initial Registration Statement automatically became effective under the Act pursuant to Rule 462(e) as of the date and time specified in such opinion, the Additional Registration Statement (if any) was filed and became effective under the Act as of the date and time (if determinable) specified in such opinion, the Final Prospectus was filed with the Commission pursuant to the subparagraph of Rule 424(b) specified in such opinion (or, if stated in such opinion, pursuant to Rule 462(c)) on the date specified therein, and, to the knowledge of such counsel, no stop order suspending the effectiveness of a Registration

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Statement or any part thereof has been issued and no proceedings for that purpose have been instituted or are pending or contemplated under the Act; the statements in the Registration Statements, General Disclosure Package and Final Prospectus of legal matters, agreements, documents or proceedings are accurate and fair summaries thereof and present the information required to be shown in all material respects; and such counsel does not know of any legal or governmental proceedings required to be described in a Registration Statement or the Final Prospectus which are not described as required or of any contracts or documents of a character required to be described in a Registration Statement or the Final Prospectus or to be filed as exhibits to a Registration Statement which are not described and filed as required.

(ix) Authorization of Agreement. This Agreement has been duly authorized, executed and delivered by the Company.

(x) Absence of Existing Defaults and Conflicts. To such counsel’s knowledge, neither the Company nor any of its subsidiaries is in violation of its declaration of trust, partnership agreement, charter or by-laws or other organizational documents and no default (or event which, with the giving of notice or lapse of time would be a default) has occurred in the due performance or observance of any material obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, loan agreement, note, lease or other agreement or instrument that is described or referred to in a Registration Statement or the General Disclosure Package or filed or incorporated by reference as an exhibit to a Registration Statement except as described in the General Disclosure Package.

(xi) Permitted Investments. The investments of the Company described in the General Disclosure Package are permitted investments under its declaration of trust.

(xii) REIT Status. The Company has qualified to be taxed as a real estate investment trust pursuant to Sections 856-860 of the Internal Revenue Code, as amended, for the fiscal years ended December 31, 2003 through December 31, 2007, and the Company’s present method of operation and its assets and contemplated income are such that the Company is in a position under present law to so qualify for the fiscal year ended December 31, 2008 and the fiscal year ending December 31, 2009, and under the present law the federal income tax treatment of the Company and its shareholders will be as set forth in the Prospectus under the heading “Description of Shares.”

(xiii) Documents Incorporated by Reference. Each document filed pursuant to the 1934 Act and incorporated by reference in the Prospectus (other than the financial statements and supporting schedules included therein or omitted therefrom, as to which such counsel need express no opinion) complied as to form when so filed in all material respects with the 1934 Act and the 1934 Act Regulations.

(xiv) Disclosure. Each Registration Statement and the Final Prospectus, and each amendment or supplement thereto, as of their respective effective or issue dates, complied as to form in all material respects with the requirements of the Act and the Rules and Regulations; such counsel has no reason to believe that any part of a Registration Statement or any amendment thereto, as of its effective date or as of such Closing Date, contained any untrue statement of a material fact or omitted to state any material fact required to be stated therein or necessary to make the statements therein not misleading or that the Final Prospectus or any amendment or supplement thereto, as of its issue date or as of such Closing Date, contained any untrue statement of a material fact or omitted to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; such counsel has

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no reason to believe that the General Disclosure Package, as of the Applicable Time or as of such Closing Date, contained any untrue statement of a material fact or omitted to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

(e) Opinion of Counsel for Underwriters. The Representatives shall have received from Vinson & Elkins LLP, counsel for the Underwriters, such opinion or opinions, dated such Closing Date, with respect to such matters as the Representatives may require, and the Company shall have furnished to such counsel such documents as they request for the purpose of enabling them to pass upon such matters. In rendering such opinion, Vinson & Elkins LLP may rely as to all matters governed by Maryland law upon the opinion of Arent Fox LLP referred to above.

(f) Officers’ Certificate. The Representatives shall have received a certificate, dated such Closing Date, of an executive officer of the Company and a principal financial or accounting officer of the Company in which such officers shall state that: the representations and warranties of the Company in this Agreement are true and correct; the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied hereunder at or prior to such Closing Date; no stop order suspending the effectiveness of any Registration Statement has been issued and no proceedings for that purpose have been instituted or, to the best of their knowledge and after reasonable investigation, are contemplated by the Commission; the Additional Registration Statement (if any) satisfying the requirements of subparagraphs (1) and (3) of Rule 462(b) was timely filed pursuant to Rule 462(b), including payment of the applicable filing fee in accordance with Rule 111(a) or (b) of Regulation S-T of the Commission; and, subsequent to the respective dates of the most recent financial statements in the General Disclosure Package, there has been no material adverse change, nor any development or event involving a prospective material adverse change, in the condition (financial or otherwise), results of operations, business, properties or prospects of the Company and its subsidiaries taken as a whole except as set forth in the General Disclosure Package or as described in such certificate.

(g) Lock-up Agreements. On or prior to the date hereof, the Representatives shall have received lockup letters from each of the executive officers and trustees of the Company.

The Company will furnish the Representatives with such conformed copies of such opinions, certificates, letters and documents as the Representatives reasonably request. The Representatives may in their sole discretion waive on behalf of the Underwriters compliance with any conditions to the obligations of the Underwriters hereunder, whether in respect of an Optional Closing Date or otherwise.

8. Indemnification and Contribution. (a) Indemnification of Underwriters. The Company will indemnify and hold harmless each Underwriter, its partners, members, directors, officers, employees, agents, affiliates and each person, if any, who controls such Underwriter within the meaning of Section 15 of the Act or Section 20 of the Exchange Act (each, an “Indemnified Party”), against any and all losses, claims, damages or liabilities, joint or several, to which such Indemnified Party may become subject, under the Act, the Exchange Act, other Federal or state statutory law or regulation or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact contained in any part of any Registration Statement at any time, any Statutory Prospectus as of any time, the Final Prospectus, the Information 8-K or any Issuer Free Writing Prospectus, or arise out of or are based upon the omission or alleged omission of a material fact required to be stated therein or necessary to make the statements therein not misleading, and will reimburse each Indemnified Party for any legal or other expenses reasonably incurred by such Indemnified Party in connection with investigating or defending against any loss, claim, damage, liability, action, litigation, investigation or proceeding whatsoever (whether or not such Indemnified Party is a party thereto), whether threatened or commenced, and in connection with the enforcement of this provision with respect to any of the above as such expenses are incurred; provided, however, that the Company will not be liable in any such case to the extent that any such loss, claim, damage or liability arises out of or is based

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upon an untrue statement or alleged untrue statement in or omission or alleged omission from any of such documents in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in subsection (b) below.

(b) Indemnification of Company. Each Underwriter will severally and not jointly indemnify and hold harmless the Company, each of its trustees and each of its officers who signs a Registration Statement and each person, if any, who controls the Company within the meaning of Section 15 of the Act or Section 20 of the Exchange Act (each, an “Underwriter Indemnified Party”), against any losses, claims, damages or liabilities to which such Underwriter Indemnified Party may become subject, under the Act, the Exchange Act, other Federal or state statutory law or regulation or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact contained in any part of any Registration Statement at any time, any Statutory Prospectus as of any time, the Final Prospectus, or any Issuer Free Writing Prospectus, or arise out of or are based upon the omission or the alleged omission of a material fact required to be stated therein or necessary to make the statements therein not misleading, in each case to the extent, but only to the extent, that such untrue statement or alleged untrue statement or omission or alleged omission was made in reliance upon and in conformity with written information furnished to the Company by such Underwriter through the Representatives specifically for use therein, and will reimburse any legal or other expenses reasonably incurred by such Underwriter Indemnified Party in connection with investigating or defending against any such loss, claim, damage, liability, action, litigation, investigation or proceeding whatsoever (whether or not such Underwriter Indemnified Party is a party thereto), whether threatened or commenced, based upon any such untrue statement or omission, or any such alleged untrue statement or omission as such expenses are incurred, it being understood and agreed that the only such information furnished by any Underwriter consists of the following information in the Final Prospectus furnished on behalf of each Underwriter: the concession figure appearing in the third paragraph under the caption “Underwriting” and the information in the eleventh through fourteenth paragraphs under the caption “Underwriting”.

(c) Actions against Parties; Notification. Promptly after receipt by an indemnified party under this Section of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against the indemnifying party under subsection (a) or (b) above, notify the indemnifying party of the commencement thereof; but the failure to notify the indemnifying party shall not relieve it from any liability that it may have under subsection (a) or (b) above except to the extent that it has been materially prejudiced (through the forfeiture of substantive rights or defenses) by such failure; and provided further that the failure to notify the indemnifying party shall not relieve it from any liability that it may have to an indemnified party otherwise than under subsection (a) or (b) above. In case any such action is brought against any indemnified party and it notifies the indemnifying party of the commencement thereof, the indemnifying party will be entitled to participate therein and, to the extent that it may wish, jointly with any other indemnifying party similarly notified, to assume the defense thereof, with counsel satisfactory to such indemnified party (who shall not, except with the consent of the indemnified party, be counsel to the indemnifying party), and after notice from the indemnifying party to such indemnified party of its election so to assume the defense thereof, the indemnifying party will not be liable to such indemnified party under this Section for any legal or other expenses subsequently incurred by such indemnified party in connection with the defense thereof other than reasonable costs of investigation. In no event shall the indemnifying parties be liable for fees and expenses of more than one counsel (in addition to any local counsel) separate from their own counsel for all indemnified parties in connection with any one action or separate but similar or related actions in the same jurisdiction arising out of the same general allegations or circumstances No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened action in respect of which any indemnified party is or could have been a party and indemnity could have been sought hereunder by such indemnified party unless such settlement (i) includes an unconditional release of such indemnified party from all liability on any claims that are the subject matter of such action and (ii) does not include a statement as to, or an admission of, fault, culpability or a failure to act by or on behalf of an indemnified party.

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(d) Contribution. If the indemnification provided for in this Section is unavailable or insufficient to hold harmless an indemnified party under subsection (a) or (b) above, then each indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of the losses, claims, damages or liabilities referred to in subsection (a) or (b) above (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other from the offering of the Offered Securities or (ii) if the allocation provided by clause (i) above is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (i) above but also the relative fault of the Company on the one hand and the Underwriters on the other in connection with the statements or omissions which resulted in such losses, claims, damages or liabilities as well as any other relevant equitable considerations. The relative benefits received by the Company on the one hand and the Underwriters on the other shall be deemed to be in the same proportion as the total net proceeds from the offering (before deducting expenses) received by the Company bear to the total underwriting discounts and commissions received by the Underwriters. The relative fault shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Company or the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such untrue statement or omission. The amount paid by an indemnified party as a result of the losses, claims, damages or liabilities referred to in the first sentence of this subsection (d) shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any action or claim which is the subject of this subsection (d). Notwithstanding the provisions of this subsection (d), no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which such Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The Underwriters’ obligations in this subsection (d) to contribute are several in proportion to their respective underwriting obligations and not joint. The Company and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 8(d) were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation which does not take account of the equitable considerations referred to in this Section 8(d).

(e) Control Persons. The obligations of the Company under this Section shall be in addition to any liability which the Company may otherwise have and shall extend, upon the same terms and conditions, to each person, if any, who controls any Underwriter within the meaning of the Act; and the obligations of the Underwriters under this Section shall be in addition to any liability which the respective Underwriters may otherwise have and shall extend, upon the same terms and conditions, to each trustee of the Company, to each officer of the Company who has signed a Registration Statement and to each person, if any, who controls the Company within the meaning of the Act.

9. Default of Underwriters. If any Underwriter or Underwriters default in their obligations to purchase Offered Securities hereunder on either the First or any Optional Closing Date and the aggregate number of Offered Securities that such defaulting Underwriter or Underwriters agreed but failed to purchase does not exceed 10% of the total number of Offered Securities that the Underwriters are obligated to purchase on such Closing Date, the Representatives may make arrangements satisfactory to the Company for the purchase of such Offered Securities by other persons, including any of the Underwriters, but if no such arrangements are made by such Closing Date, the non-defaulting Underwriters shall be obligated severally, in proportion to their respective commitments hereunder, to purchase the Offered Securities that such defaulting Underwriters agreed but failed to purchase on such Closing Date. If any Underwriter or Underwriters so default and the aggregate number of Offered Securities with respect to

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which such default or defaults occur exceeds 10% of the total number of Offered Securities that the Underwriters are obligated to purchase on such Closing Date and arrangements satisfactory to the Representatives and the Company for the purchase of such Offered Securities by other persons are not made within 36 hours after such default, this Agreement will terminate without liability on the part of any non-defaulting Underwriter or the Company, except as provided in Section 10. As used in this Agreement, the term “Underwriter” includes any person substituted for an Underwriter under this Section. Nothing herein will relieve a defaulting Underwriter from liability for its default.

10. Survival of Certain Representations and Obligations. The respective indemnities, agreements, representations, warranties and other statements of the Company or its officers and of the several Underwriters set forth in or made pursuant to this Agreement will remain in full force and effect, regardless of any investigation, or statement as to the results thereof, made by or on behalf of any Underwriter, the Company or any of their respective representatives, officers, directors or trustees or any controlling person, and will survive delivery of and payment for the Offered Securities. If the purchase of the Offered Securities by the Underwriters is not consummated for any reason other than solely because of the termination of this Agreement pursuant to Section 9 hereof, the Company will reimburse the Underwriters for all out-of-pocket expenses (including fees and disbursements of counsel) reasonably incurred by them in connection with the offering of the Offered Securities, and the respective obligations of the Company and the Underwriters pursuant to Section 8 hereof shall remain in effect. In addition, if any Offered Securities have been purchased hereunder, the representations and warranties in Section 2 and all obligations under Section 5 shall also remain in effect.

11. Notices. All communications hereunder will be in writing and, if sent to the Underwriters, will be mailed, delivered or telegraphed and confirmed to the Representatives c/o Wachovia Capital Markets, LLC, 375 Park Avenue, New York, New York 10152, attention: Equity Syndicate Department, fax ###-###-####; Citigroup Global Markets Inc., 388 Greenwich Street, New York, New York 10013, attention: General Counsel; and to Raymond James & Associates, Inc., 880 Carrillon Parkway, St. Petersburg, Florida 33716, attention: General Counsel; or, if sent to the Company, will be mailed, delivered or telegraphed and confirmed to it at 6110 Executive Boulevard, Suite 800, Rockville, Maryland 20852, Attention: William Camp, Executive Vice President and Chief Financial Officer.

12. Successors. This Agreement will inure to the benefit of and be binding upon the parties hereto and their respective successors and the officers, trustees and directors and controlling persons referred to in Section 8, and no other person will have any right or obligation hereunder.

13. Representation of Underwriters. The Representatives will act for the several Underwriters in connection with this financing, and any action under this Agreement taken by the Representatives will be binding upon all the Underwriters.

14. Counterparts. This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, but all such counterparts shall together constitute one and the same Agreement.

15. Absence of Fiduciary Relationship. The Company acknowledges and agrees that:

(a) No Other Relationship. The Underwriters have been retained solely to act as underwriters in connection with the sale of Offered Securities and that no fiduciary, advisory or agency relationship between the Company and the Underwriters have been created in respect of any of the transactions contemplated by this Agreement or the Final Prospectus, irrespective of whether the Underwriters have advised or are advising the Company on other matters;

(b) Arms’ Length Negotiations. The price of the Offered Securities set forth in this Agreement was established by the Company following discussions and arms-length negotiations with the Underwriters and the Company is capable of evaluating and understanding and understands and accepts the terms, risks and conditions of the transactions contemplated by this Agreement;

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(c) Absence of Obligation to Disclose. The Company has been advised that the Underwriters and their affiliates are engaged in a broad range of transactions which may involve interests that differ from those of the Company and that the Underwriters have no obligation to disclose such interests and transactions to the Company by virtue of any fiduciary, advisory or agency relationship; and

(d) Waiver. The Company waives, to the fullest extent permitted by law, any claims it may have against the Underwriters for breach of fiduciary duty or alleged breach of fiduciary duty and agrees that the Underwriters shall have no liability (whether direct or indirect) to the Company in respect of such a fiduciary duty claim or to any person asserting a fiduciary duty claim on behalf of or in right of the Company, including shareholders, employees or creditors of the Company.

16. Applicable Law. This Agreement shall be governed by, and construed in accordance with, the laws of the State of New York.

The Company hereby submits to the non-exclusive jurisdiction of the Federal and state courts in the Borough of Manhattan in The City of New York in any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby. The Company irrevocably and unconditionally waives any objection to the laying of venue of any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby in Federal and state courts in the Borough of Manhattan in The City of New York and irrevocably and unconditionally waives and agrees not to plead or claim in any such court that any such suit or proceeding in any such court has been brought in an inconvenient forum.

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If the foregoing is in accordance with the Representatives’ understanding of our agreement, kindly sign and return to the Company one of the counterparts hereof, whereupon it will become a binding agreement between the Company and the several Underwriters in accordance with its terms.

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SCHEDULE A

SCHEDULE B

“General Use Issuer Free Writing Prospectus” includes each of the following documents:

None

The following information is also included in the General Disclosure Package:

The initial price to the public of the Offered Securities which is $21.40.

The number of Firm Securities to be purchased by the underwriters is 5,000,000.

SCHEDULE C

The Representatives shall have received letters, dated, respectively, the date hereof and the First Closing Date, of Ernst & Young LLP confirming that Ernst & Young LLP is a registered public accounting firm and independent public accountants within the meaning of the Securities Laws to the effect that:

(i) in their opinion the audited consolidated financial statements and schedules examined by them and included or incorporated by reference in the Registration Statement and the General Disclosure Package comply as to form in all material respects with the applicable accounting requirements of the Securities Laws;

(ii) with respect to the period(s) covered by the unaudited quarterly consolidated financial statements included in the Registration Statements and the General Disclosure Package, they have performed the procedures specified by the American Institute of Certified Public Accountants for a review of interim financial information as described in AU 722, Interim Financial Information, on the unaudited quarterly consolidated financial statements (including the noted thereto) of the Company and its consolidated subsidiaries included in the Registration Statement and the General Disclosure Package, and have made inquiries of certain officials of the Company who have responsibility for financial and accounting matters of the Company and its consolidated subsidiaries as to whether such unaudited quarterly consolidated financial statements comply as to form in all material respects with the applicable accounting requirements of the Securities Act and the related published rules and regulations; they have read the latest unaudited monthly consolidated financial statements (including the notes thereto) and the supplementary summary unaudited financial information of the Company and its consolidated subsidiaries made available by the Company and the minutes of the meetings of the shareholders, Board of Trustees and committees of the Board of Trustees of the Company; and have made inquiries of certain officials of the Company who have responsibility for financial and accounting matters of the Company and its consolidated subsidiaries as to whether the unaudited monthly financial statements are stated on a basis substantially consistent with that of the audited consolidated financial statements included in the Registration Statement and General Disclosure Package; and on the basis thereof, nothing came to their attention which caused them to believe that:

(A) the unaudited financial statements included in the Registration Statements or the General Disclosure Package do not comply as to form in all material respects with the applicable accounting requirements of the Securities Laws, or that any material modifications should be made to the unaudited quarterly consolidated financial statements for them to be in conformity with generally accepted accounting principles;

(B) with respect to the period subsequent to the date of the most recent unaudited quarterly consolidated financial statements included in the Registration Statements or the General Disclosure Package, at a specified date at the end of the most recent month, there were any increases in the short-term debt or long-term debt of the Company and its consolidated subsidiaries, or any change in shareholders’ equity or the consolidated capital stock of the Company and its consolidated subsidiaries or any decreases in the net current assets or net assets of the Company and its consolidated subsidiaries, as compared with the amounts shown on the latest balance sheet included in the General Disclosure Package; or for the period from the day after the date of the most recent unaudited quarterly consolidated financial statements included in the General

Disclosure Package for such entities to such specified date, there were any decreases, as compared with the corresponding period in the preceding year, in real estate rental revenue, net income or net income per share of the Company and its consolidated subsidiaries, except for such changes, increases or decreases set forth in such letter which the General Disclosure Package discloses have occurred or may occur;

(iii) With respect to any period as to which officials of the Company have advised that no consolidated financial statements as of any date or for any period subsequent to the specified date referred to in (ii)(B) above are available, they have made inquiries of certain officials of the Company who have responsibility for the financial and accounting matters of the Company and its consolidated subsidiaries as to whether, at a specified date not more than three business days prior to the date of such letter, there were any increases in the short-term debt or long-term debt of the Company and its consolidated subsidiaries, or any change in shareholders’ equity or the consolidated capital stock of the Company and its consolidated subsidiaries or any decreases in the net current assets or net assets of the Company and its consolidated subsidiaries, as compared with the amounts shown on the most recent balance sheet for such entities included in the General Disclosure Package; or for the period from the day after the date of the most recent unaudited quarterly financial statements for such entities included in the General Disclosure Package to such specified date, there were any decreases, as compared with the corresponding period in the preceding year, in real estate rental revenue, net income or net income per share of the Company and its consolidated subsidiaries and, on the basis of such inquiries and the review of the minutes described in paragraph (ii) above, nothing came to their attention which caused them to believe that there was any such change, increase, or decrease, except for such changes, increases or decreases set forth in such letter which the General Disclosure Package discloses have occurred or may occur;

(iv) they have compared specified dollar amounts (or percentages derived from such dollar amounts) and other financial and statistical information contained in the Registration Statement, each Issuer Free Writing Prospectus (other than any Issuer Free Writing Prospectus that is an “electronic road show,” as defined in Rule 433(h)) and the General Disclosure Package (in each case to the extent that such dollar amounts, percentages and other financial and statistical information are derived from the general accounting records of the Company and its subsidiaries or are derived directly from such records by analysis or computation) with the results obtained from inquiries, a reading of such general accounting records and other procedures specified in such letter and have found such dollar amounts, percentages and other financial and statistical information to be in agreement with such results; and

(v) based upon limited procedures set forth in detail in such letter, nothing came to their attention that caused them to believe that the unaudited pro forma consolidated financial statements included or incorporated by reference in the Registration Statement and the General Disclosure Package do not comply as to form in all material respects with the applicable accounting requirements of Rule 11-02 of Regulation S-X or that the pro forma adjustments have not been properly applied to the historical amounts in the compilation of those statements.

For purposes of this Schedule, if the Effective Time of the Additional Registration Statement is subsequent to the execution and delivery of this Agreement, “Registration Statements” shall mean the Initial Registration Statement and the Additional Registration Statement as proposed to be filed shortly prior to its Effective Time, and “Prospectus” shall mean the prospectus included in the Registration Statements. All financial statements and schedules included in material incorporated by reference into the Prospectus shall be deemed included in the Registration Statements for purposes of this Schedule.