D.C. Mun. Regs. tit. 11, r. 11-G305
A minimum rear yard of two and one-half inches (2.5 in.) per one foot (1 ft.) of vertical distance from the mean finished grade at the middle of the rear of the structure to the highest point of the main roof or parapet wall, but not less than twelve feet (12 ft.) shall be required above a horizontal place as described in Subtitle G § 305.2 in the MU-1 and MU-2 zones.
A horizontal plane may be established at twenty feet (20 ft.) above the mean finished grade at the middle of the rear of the structure for the purposes of measuring rear yards.
...
A rear yard is not required to be provided below a horizontal plane as described in Subtitle G § 305.2.
Where a lot abuts an alley, the rear yard may be measured from the center line of the alley to the rear wall of the building or other structure.
Where a lot does not abut an alley, the rear yard shall be measured from the rear lot line to the rear wall of the building or other structure.
D.C. Mun. Regs. tit. 11, r. 11-G305