D.C. Code § 16-1331: Acquisition of property in excess of needs.

In order to promote the orderly and proper development of the seat of government of the United States, the Mayor of the District of Columbia may acquire, in the public interest, by gift, dedication, exchange, purchase, or condemnation, fee simple title to land or rights in, or on land, or easements or restrictions therein, within the District, for public uses, works, and improvements authorized by Congress, in excess of that actually needed for and essential to their usefulness, in order to preserve the view, appearance, light, and air and to enhance their usefulness to prevent the use of private property adjacent to them in such a manner as to impair the public benefit derived from the construction thereof, or to prevent inequities or hardship to the owners of adjacent private property by depriving them of the beneficial use of their property.