D.C. Mun. Regs. r. 11-2000
The provisions of this title shall establish separate districts, each of which is an appropriate area for the location of uses and structures permitted in that district.
Within the districts established by this title, or in amendments that may later be adopted, there exist structures, uses of land, and uses of structures that were lawful before this title was adopted or amended, but that would be prohibited, regulated, or restricted under the terms of this title or future amendments to this title. It is the intent of this title that nonconformities may not be enlarged upon, expanded, or extended, nor may they be used as a basis for adding other structures or uses prohibited elsewhere in the same district.
It is necessary and consistent with the establishment of the separate districts under this title that all uses and structures incompatible with permitted uses or structures shall be regulated strictly and permitted only under rigid controls, to the extent permitted by the Zoning Act of 1938, approved June 20, 1938 (52 Stat, 797, as amended; D.C. Official Code §§ 6-641.01 to 641.15 (2001)(formerly codified at D.C. Code §§ 5-413 to 5-432 (1994 Repl. & 1999 Supp.))).
Any nonconforming use of a structure or of land, or any nonconforming structure lawfully existing on May 12, 1958, that remains nonconforming, and any use or structure lawfully existing that became nonconforming on May 12, 1958, may be continued, operated, occupied, or maintained, subject to the provisions of this chapter.
Nonconformities shall be regulated in the following categories:
A nonconforming use of land or of land with structures incidental to the use of the land shall neither be extended in land area nor changed to any use except a use permitted in the district in which the property is located.
D.C. Mun. Regs. r. 11-2000