D.C. Mun. Regs. tit. 11, r. 11-E208
Except as provided elsewhere in this title, the minimum side yard shall be as set forth in this section.
Two (2) side yards shall be provided for detached buildings; one (1) side yard shall be provided for semi-detached buildings; and no side yards are required for row buildings.
Any side yard provided shall be a minimum of five feet (5 ft.).
Existing conforming side yards shall not be reduced to a nonconforming width or eliminated.
In the case of a building with a nonconforming 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.).
In the case of a lot proposed to be used by a public library or public recreation and community center where a side lot line abuts or adjoins a public open space, recreation area, or reservation, no side yard shall be required.
D.C. Mun. Regs. tit. 11, r. 11-E208