D.C. Mun. Regs. tit. 11, r. 11-B318
In the case of a lot that is triangular or irregularly shaped, the furthermost point or line from the street lot line shall be deemed the point or line from which the required rear yard shall be measured.
The depth of a required rear yard shall be measured as the mean horizontal distance between the rear line of a building and the rear lot line, except as provided elsewhere in this title.
Where the rear lot line is not parallel to the street lot line, or where there are more than one (1) rear lot lines that intersect at a point at an angle greater than ninety degrees (90º), the required rear yard shall be measured as a vertical plane along a line measured in from the rear lot line at a point equidistant from the side lot lines;
Where a lot does not have a rear lot line, such as when the side yards converge at a point, or where the rear lot lines intersect at an angle less than ninety degrees (90º), a rear yard shall be measured as an arc from the point opposite the front lot line(s); and
Where there is more than one (1) rear lot line generally parallel to the front lot line but separated by a lot line generally perpendicular to the rear lot lines, then the rear yard setback shall be measured from the rear lot line more distant from the front lot line, and measured across the full width of the property to where it intersects both side lot lines.
In the case of a corner lot in any D zone other than the D-1-R zone, a court complying with the width requirements for a closed court as specified in Subtitle I § 207.1 and Subtitle B § 324 may be provided in lieu of a rear yard. For the purposes of this section, the required court shall be provided above a horizontal plan twenty-five (25) feet above the mean elevation of the rear lot line.
In the case of a corner lot in the MU-1, MU-2, MU-8, MU-9, MU-15, NMU-8B/H-H, and CG-3 zones, a court complying with the width requirements for a closed court as applicable for each zone may be provided in lieu of a rear yard. For the purposes of this section, the required court shall be provided above a horizontal plan beginning not more than twenty feet (20 ft.) above the curb grade opposite the center of the front of the building and the width of the court shall be computed for the entire height of court.
In the case of a through or corner lot abutting three (3) or more streets, the depth of rear yard may be measured from the center line of the street abutting the lot at the rear of the structure.
D.C. Mun. Regs. tit. 11, r. 11-B318