Washington DC accessory apartment (ADU) rules

Under 11 DCMR Subtitle U section 253, one accessory apartment may be established in an R zone. It is permitted in a principal dwelling or accessory building as a matter of right in the R zones, except the R-19 and R-20 zones, where it requires a special exception approved by the Board of Zoning Adjustment. An in-house ADU may not occupy more than thirty-five percent of the house's gross floor area, no more than three persons may occupy it, and either the principal dwelling or the ADU must be owner-occupied. The Zoning Administrator at the DC Department of Buildings reviews permits.

Building or renting an unpermitted accessory apartment, exceeding the three-person occupancy cap, or breaking the owner-occupancy condition draws zoning enforcement, stop-work orders, and civil fines from the DC Department of Buildings.