D.C. Mun. Regs. tit. 11, r. 11-F606
No side yards are required in the RA-8, RA-9, and RA-10 zones; however, if a side yard is provided, it shall be a minimum of four feet (4 ft.).
Side yards for a detached or semi-detached building containing one (1) or two (2) dwelling units in the RA-8, RA-9, and RA-10 zones shall be a minimum of eight feet (8 ft.).
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A side yard shall not be required along a side street abutting a corner lot in the RA-8, RA-9, and RA-10 zones.
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-F606