An addition to a one-family dwelling or flat, in those Residence Districts where a flat is permitted, or a new or enlarged accessory structure on the same lot as a one-family dwelling or flat, shall be permitted even though the addition or accessory structure does not comply with all of the requirements of §§ 401, 403, 404, 405, 406, and 2001.3 shall be permitted as a special exception if approved by the Board of Zoning Adjustment under § 3104, subject to the provisions of this section.
The addition or accessory structure shall not have a substantially adverse affect on the use or enjoyment of any abutting or adjacent dwelling or property, in particular:
The lot occupancy of all new and existing structures on the lot shall not exceed fifty percent (50%) in the R-1 and R-2 Districts or seventy percent (70%) in the R-3, R-4, and R-5 Districts.
The Board may require special treatment in the way of design, screening, exterior or interior lighting, building materials, or other features for the protection of adjacent or nearby properties.
This section may not be used to permit the introduction or expansion of a nonconforming use .
D.C. Mun. Regs. tit. 11, r. 11-223