D.C. Mun. Regs. tit. 11, r. 11-F406
A minimum of one (1) side yard shall be provided for all buildings unless the building contains three (3) or more dwelling units per floor, in which case two (2) side yards shall be provided, each with the minimum distance equal to three inches (3 in.) per foot of building height but not less than eight feet (8 ft.).
Side yards for a detached or semi-detached building containing one (1) or two (2) dwelling units shall be a minimum of eight feet (8 ft.) in the RA-6 zone.
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A side yard shall not be required along a side street abutting a corner lot.
Existing conforming side yards may not be reduced to a non-conforming width or eliminated.
In the case of a building with a non-conforming side yard, an extension or addition may be made to the building; provided, that the width of the existing side yard shall not be reduced or eliminated; and provided further, that the width of the side yard adjacent to the extension or addition shall be a minimum of three feet (3 ft.).
D.C. Mun. Regs. tit. 11, r. 11-F406