D.C. Mun. Regs. tit. 11, r. 11-C201
Except as otherwise permitted in this chapter, nonconforming structures or uses 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 zone district.
Any nonconforming use of a structure or of land, or any nonconforming structure lawfully existing on the effective date of this title that remains nonconforming, and any use or structure lawfully existing that became nonconforming on the effective date of this title, may be continued, operated, occupied, or maintained, subject to the provisions of this chapter.
It is necessary and consistent with the establishment of the separate zone 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.
D.C. Mun. Regs. tit. 11, r. 11-C201