PROMISSORY NOTE

Exhibit 99.6

PROMISSORY NOTE

Subject to the terms and conditions of this Promissory Note (this “ Note ”), for value received, ACADIA BRANDYWINE CONDOMINIUM, LLC, a Delaware limited liability company (the “ Borrower ”), hereby promises to pay to the order of ACADIA REALTY LIMITED PARTNERSHIP, a Delaware limited partnership (“ Payee ”), at Payee’s offices located at c/o Acadia Realty Trust 1311 Mamaroneck Avenue, Suite 260, White Plains, New York 10605 (or at such other place as Payee designates in writing to the Borrower), in lawful money of the United States of America, the principal sum of SIXTEEN MILLION NINE HUNDRED FIFTEEN THOUSAND DOLLARS ($16,915,000), together with interest at a rate equal to nine percent (9%) per annum. This Note is issued by the Borrower pursuant to that certain Agreement and Plan of Merger, dated as December 22, 2005, by and among Acadia Realty Acquisition I, LLC, ARA BTC LLC, ARA MS LLC, ARA BS LLC, ARA BC LLC, ARA BH LLC, Acadia Investors, Inc., AII BTC LLC, AII MS LLC, AII BS LLC, AII BC LLC, AII BH LLC, Samuel Ginsberg 2000 Trust Agreement #1, Martin Ginsberg 2000 Trust Agreement #1, Martin Ginsburg, Samuel Ginsburg, Adam Ginsburg, GDC SMG, LLC, GDC Beechwood, LLC, Aspen Cove Apartments, LLC and SMG Celebration, LLC (as amended, restated or otherwise modified from time to time, the “ Merger Agreement ”). Capitalized terms used and not defined herein shall have the meaning given to them in the Merger Agreement. This Note is secured by and entitled to the benefits of the AII Bridge Mortgage of the Borrower.

SECTION 1

PAYMENT

1.1          Maturity Date . All outstanding principal and accrued and unpaid interest on this Note shall be due and payable in full, and this Note shall mature, on July 3, 2006 (the “ Maturity Date ”).

1.2          Interest . This Note shall bear interest on the outstanding principal amount hereof from and including the date of this Note. Interest shall be calculated on the basis of a 365-day or 366-day year, as the case may be, and charged for the actual number of days elapsed.

1.3          Optional Prepayments . The Borrower may prepay the principal amount of this Note, in whole or in part (together with all accrued interest thereon as of the date of such prepayment), at any time or from time to time, without penalty. Notwithstanding the foregoing, the Borrower shall not make any optional prepayment of this Note unless it shall contemporaneously therewith make an optional prepayment of a pro-rata amount of the other ARA Bridge Notes and the AII Bridge Notes, with such pro-rata amount determined on the basis of the aggregate principal amount of the other ARA Bridge Notes, the AII Bridge Notes and this Note.

1.4         Payment Method . Any payment of principal and interest on this Note will be made in currency of the United States of America by check mailed or by wire transfer of immediately available funds to an account designated by Payee in writing.

SECTION 2

COVENANTS

So long as any principal of, or accrued interest on, this Note is outstanding or any other obligation of the Borrower hereunder shall remain unpaid or unsatisfied:

2.1           Indebtedness . The Borrower shall not directly or indirectly create, incur, assume, extend the maturity of, or otherwise become directly or indirectly liable with respect to, any Indebtedness other than (a) Indebtedness evidenced by this Note, (b) Indebtedness incurred in connection with any refinancing of Indebtedness evidenced by this Note, the other ARA Bridge Notes and the AII Bridge Notes, provided , however , that the Borrower shall not refinance the Indebtedness evidenced by the AII Bridge Notes or the other ARA Bridge Notes unless simultaneously therewith the Borrower refinances all of the Indebtedness evidenced by this Note, (c) Indebtedness owing to Payee or any of its Affiliates and (d) any other Indebtedness if (i) prior to incurring such Indebtedness the Borrower shall give written notice of the proposed Indebtedness (the “ Notice of Intention ”) to Payee, specifying the amount, interest rate, term and all other material terms and conditions of the proposed Indebtedness, and granting to Payee (for a period of thirty (30) days following its receipt of the Notice of Intention) an irrevocable right to make the loans and enter into commercially reasonable agreements with respect to such Indebtedness on the other terms specified in the Notice of Intention, and (ii) Payee does not exercise such right by delivery of notice to the Borrower within such 30-day period.

2.2           Liens . The Borrower shall not directly or indirectly, create, incur, assume or permit to exist any Liens upon or with respect to any of their respective assets, whether now owned hereafter acquired or any income or profits therefrom, or assign or otherwise convey any right to receive income to secure any Indebtedness, other than (a) Liens existing as of the Closing under the Merger Agreement and (b) Liens created under the ARA Bridge Mortgage of the Borrower.

2.3           Merger, Consolidation, Sale of Assets . The Borrower shall not (a) liquidate, wind-up or dissolve, (b) consolidate or merge with or into any other Person, (c) permit any other Person to consolidate with or merge with or into it or participate in a share exchange with it or (d) sell, lease, transfer, contribute or otherwise dispose of any of its assets to any other Person (other than sales of immaterial and worn out and obsolete assets in the ordinary course of business).

2.4           Loans and Investments . The Borrower shall not (a) make any loan, advance or capital contribution, (b) extend credit to any Person or (c) purchase or otherwise acquire, hold or invest in, or make any commitment to purchase or otherwise acquire, hold or invest in, any debentures, notes and other evidences of Indebtedness, stocks, securities (including rights to purchase and securities convertible into or

exchangeable for other securities), interests in joint ventures and general and limited partnerships or limited liability companies, mortgage loans and other investment or portfolio assets (owned of record or beneficially by the Borrower), or make or keep outstanding any advance or loans, except in accordance with Section 2.11 .

2.5           Dividends, Etc . After the occurrence and during the continuance of an Event of Default, the Borrower shall not declare or pay any cash or asset dividend on any of its membership interests or make any other distribution or disposition of any assets to members in respect of its membership interests (or otherwise), or make, or commit to make, any payment on account of the purchase, redemption or other retirement of any of its membership interests or other equity securities or options, warrants or convertible securities therefor.

2.6           Subsidiaries . The Borrower shall not organize or cause to exist any subsidiary of the Borrower.

2.7           Certain Limitations . The Borrower shall not enter into any agreement with any Person (other than ARA or AII) that prohibits or limits the ability of the Borrower to create, incur, assume or suffer to exist any Lien upon any of the assets or revenues of the Borrower, whether now owned or hereafter acquired.

2.8           Conflicting Agreements . The Borrower shall not enter into any agreements or arrangements that could reasonably be considered to be not in the normal course of business and which by their terms (or reasonably foreseeable effect) materially restrict or materially adversely affect the Borrower’s right and ability to meet its obligations Payee hereunder.

2.9           Affiliate Transactions . The Borrower shall not enter into any transaction with any Affiliate (other than ARA, AII and their respective Affiliates) or any of the other property Owners.

2.10        Change in Nature of Business . The Borrower shall not engage in any business other than the busin