D.C. Mun. Regs. tit. 11-B, § 203
203.1 Any use allowed as a permitted use in a zone shall be allowed as an accessory use within that zone.
203.2 Any use allowed only with conditions in a zone shall be allowed as an accessory use within that zone, subject to all applicable conditions.
203.3 Accessory uses shall:
(a) Be customarily incidental and subordinate to the principal use, and located on the same lot with the principal use; and
(b) Meet all of the conditions of the appropriate use category.
SOURCE: Final Rulemaking published at 63 DCR 2447, 2650 (March 4, 2016 – Part 2).