D.C. Code § 6-641.06a
The lawful use of a building or premises as existing and lawful at the time of the original adoption of any regulation heretofore adopted under the authority of § 6-621.01, or, in the case of any regulation adopted after June 20, 1938, under this subchapter, at the time of such adoption, may be continued although such use does not conform with the provisions of such regulation, provided no structural alteration, except such as may be required by law or regulation, or no enlargement is made or no new building is erected. The Zoning Commission may in its discretion provide, upon such terms and conditions as may be set forth in the regulations, for the extension of any such nonconforming use throughout the building and for the substitution of nonconforming uses.
D.C. Code § 6-641.06a