D.C. Mun. Regs. tit. 11, r. 11-D208
Except as provided elsewhere in this title, the minimum side yard requirements shall be as set forth in this section.
Two (2) side yards, each a minimum of eight feet (8 ft.) in width, shall be provided for all detached buildings.
In any of the R-2 zones, one (1) side yard, a minimum of eight feet (8 ft.) in width, shall be provided for all semi-detached buildings.
In any of the R-3 zones, one (1) side yard, a minimum of five feet (5 ft.) in width, shall be provided for all semi-detached buildings.
No side yards are required for row buildings. An existing detached or semidetached building shall not be treated as a row building through construction or additions.
Existing conforming side yards shall 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 five feet (5 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-D208