D.C. Mun. Regs. tit. 11, r. 11-D4908
Two (2) side yards, each a minimum of eight feet (8 ft.) in width, shall be provided in all R-1 zones.
In all R-2 zones, one (1) side yard, a minimum of eight feet (8 ft.) in width, shall be provided for all semi-detached buildings and two (2) side yards, each a minimum of eight feet (8 ft.) in width, shall be provided for all detached buildings.
In all R-3 zones, a side yard shall not be required. However, except as provided in Subtitle D §§ 4908.4 and 4908.5, if the yard is provided, it shall be not less than five feet (5 ft.) wide.
In the case of a lot that abuts or adjoins a public open space, recreation area, or reservation on one (1) or more side lot line, a required side yard may be reduced or omitted.
A side yard may be reduced or omitted along a side street abutting a corner lot in an R zone.
D.C. Mun. Regs. tit. 11, r. 11-D4908