D.C. Mun. Regs. tit. 11, r. 11-B308
The height of buildings, not including a penthouse or rooftop structure, in residential zones, as defined in Subtitle A § 101.9, shall be measured in accordance with the rules provided in this section. If more than one (1) of these subsections applies to a building, the rule permitting the greater height shall apply.
The building height measuring point (BHMP) shall be established at the adjacent natural or finished grade, whichever is the lower in elevation, at the mid-point of the building façade of the principal building that is closest to a street lot line. For any excavations projecting from the building's façade other than an exception to grade as defined at Subtitle B § 100.2 the elevation of the midpoint of a building façade shall be the equivalent of the lowest such elevation; excluding existing driveways adjacent to the midpoint(s) directly connecting a garage and public right of way.
The height of a building with a flat roof shall be measured from the BHMP to the highest point of the roof excluding parapets and balustrades not exceeding four feet (4 ft.) in height.
The height of a building permitted to be ninety feet (90 ft.) shall be measured from the BHMP to the highest point of the roof excluding parapets and balustrades not exceeding four feet (4 ft.) in height.
Where a building is removed from all lot lines by a distance equal to its proposed height above grade, the height of building shall be measured from the BHMP to the highest point of the roof or parapet.
If a building fronts on more than one (1) street, any front may be used to determine street frontage; but the basis for measuring the height of the building shall be established by the street selected as the front of the building.
A conforming structure in existence on June 14, 2013, that would have been rendered nonconforming as a result of the adoption of amendments to this section made in Z.C. Order No. 12-11, shall be deemed conforming; provided that the height of the structure may neither be increased or extended.
For Alley Lots, the BHMP shall be established at grade at the mid-point of the Alley Lot Line or, where an Alley Lot abuts more than one alley, the mid-point of the alley lot line that would result in the BHMP with the highest elevation. Building height for Alley Lots shall be measured in accordance with Subtitle B §§ 308.2 through 308.4 and the rules provided in the applicable zone district, with any conflict resolved in favor of the lowest maximum height.
The building height of accessory buildings shall be measured from (i) the lower of the finished or natural grade at the middle of the side of the accessory building that faces the principal building, or (ii) the midpoint of the alley lot line if required for vehicular access to the alley, to the highest point of the roof of the building including parapets, penthouses, and rooftop structures.
D.C. Mun. Regs. tit. 11, r. 11-B308