D.C. Code § 47-844
The Council may provide that the owners of historic buildings which have been so designated by the Joint Committee on Landmarks of the National Capital may enter into agreements with the government of the District of Columbia for periods of at least 20 years which will assure the continued maintenance of historic buildings in return for property tax relief. Such a provision shall, as a condition for tax relief, require reasonable assurance that such buildings will be used and properly maintained and such other conditions as the Council finds to be necessary to encourage the preservation of historic buildings. The Council shall also provide for the recovery of back taxes, with interest, which would have been due and payable in the absence of the exemption, if the conditions for such exemption are not fulfilled.
D.C. Code § 47-844