Exhibit 5.1

Stephanie Heim

Vice President and Counsel

PS Business Parks, Inc.

701 Western Avenue

Glendale, California 91201-2397

May 3, 2012

PS Business Parks, Inc.

701 Western Avenue

Glendale, California 91201-2397

Ladies and Gentlemen:

As Vice President and Counsel of PS Business Parks, Inc. (the “Company”), I have acted as counsel to the Company in connection with the issuance and sale (the “Offering”) of up to 16,100,000 depositary shares (the “Shares”), each representing 1/1,000^th of a share of the Company’s 6.00% Cumulative Preferred Stock, Series T (the “Preferred Shares”), pursuant to the terms of the Underwriting Agreement dated May 3, 2012 (the “Agreement”) among Merrill Lynch, Pierce, Fenner & Smith Incorporated, Morgan Stanley & Co. LLC and Wells Fargo Securities, LLC, as representatives of the underwriters named in the Agreement, and the Company and PS Business Parks, L.P. All of the Shares are to be sold by the Company pursuant to a prospectus supplement dated May 3, 2012 and the accompanying prospectus dated March 13, 2012 (such documents, collectively, the “Prospectus”) that form a part of the Company’s registration statement on Form S-3 (File No. 333-180058) which was declared effective by the Securities and Exchange Commission on March 13, 2012 (the “Registration Statement”). This opinion is rendered in connection with the Offering of the Shares and in connection with the filing of a Current Report on Form 8-K relating to the Offering of the Shares, which Form 8-K will be incorporated by reference into the Registration Statement and Prospectus.

I am familiar with the proceedings taken or to be taken by the Company relating to the authorization and issuance of the Shares and the Preferred Shares. For purposes of this opinion letter, I have also examined copies of such agreements, instruments and documents, including without limitation, the Prospectus, the Registration Statement, the Deposit Agreement dated May 3, 2012 between the Company and American Stock Transfer & Trust Company, LLC, as depositary (the “Deposit Agreement”), the Company’s Restated Articles of Incorporation and Restated Bylaws, and have examined such other documents and made such other investigation as I have deemed necessary to express the opinions contained herein. In my examination of the aforesaid documents, I have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to me as copies. I have also assumed that the Shares will not be issued in violation of the ownership limit contained in the Company’s Articles of Incorporation.

Based upon, subject to and limited by the foregoing, it is my opinion that:

1. The Preferred Shares, upon receipt by the Company of the consideration for the Preferred Stock specified in the resolutions of the Board of Directors, will be validly issued, fully paid and non-assessable.

2. The depositary receipts evidencing the Shares, upon due countersignature thereof and issuance against a deposit of the Preferred Shares in accordance with the Deposit Agreement, will be validly issued and entitle the holders thereof to the rights specified in such depositary receipts and Deposit Agreement.

This opinion is based as to matters of law solely on applicable provisions of the following, as currently in effect: (1) as to the opinion given in paragraph 1, the applicable provisions of the General Corporation Law of the State of California, and (2) as to the opinions given in paragraph 2, the laws of the State of New York (but not including any laws, statutes, ordinances, administrative decisions, rules or regulations of any political subdivision of New York.) I express no opinion herein as to any other laws, statues, ordinances, rules or regulations. I hereby consent to the reference to me under the “Legal Matters” in the Prospectus and to the filing of this opinion as Exhibit 5.1 to a Current Report on Form 8-K filed in connection with the Offering. In giving this consent, I do not thereby admit that I am an “expert” within the meaning of the Securities Act of 1933, as amended.