D.C. Mun. Regs. tit. 11, r. 11-B307
In other than residential zones, as defined in Subtitle A § 101.9, and except as permitted elsewhere in this section and the regulations, the building height measuring point (BHMP) shall be established at the level of the curb, opposite the middle of the front of the building, and the building height shall be the vertical distance measured from the BHMP to the highest point of the roof or parapet or to a point designated by a specific zone district; except that Alley Lots shall be regulated by Subtitle B § 307.8.
Unless otherwise restricted or permitted in this title, in those zones in which the height of the building is limited to forty feet (40 ft.), the BHMP may be established at the adjacent natural or finished grade, whichever is the lower in elevation, at the middle of the front of the building and building height shall be measured from the BHMP to the ceiling of the top story.
In those zones in which the height of the building is limited to sixty feet (60 ft.), in the case of a building located upon a terrace, the height of the building may be measured from the top of the terrace to the highest point of the roof or parapet, but the allowance for terrace height shall not exceed five feet (5 ft.).
Except as provided in Subtitle B § 307.6, where a building is removed from all lot lines by a distance equal to its proposed height above grade, the BHMP shall be established at the adjacent natural or finished grade, whichever is the lower in elevation, at the middle of the front of the building to the highest point of the roof or parapet.
Except as provided in Subtitle B § 307.7, if a building fronts on more than one (1) street, any front may be used to determine the maximum height of the building; but the basis for the height of the building shall be determined by the width of the street selected as the front of the building.
In those zones in which the height of a building is permitted to be ninety feet (90 ft.) or greater, the BHMP shall be established at the level of the curb, opposite the middle of the front of the building and the building height shall be measured from the BHMP to the highest point of the roof excluding parapets not exceeding four feet (4 ft.) in height.
The term "curb" shall refer to a curb at grade. When the curb grade has been artificially changed by a bridge, viaduct, embankment, ramp, abutment, excavation, tunnel, or other type of artificial elevation or depression, the BHMP shall be established using the first of the following four (4) methods that is applicable to the site:
For Alley Lots, the BHMP shall be established at grade at the mid-point of the Alley Lot Lineor, 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 the vertical distance measured from the BHMP to the highest point of the roof or parapet or to a point designated by the rules provided in the applicable zone district, with any conflict resolved in favor of the lowest maximum height.
D.C. Mun. Regs. tit. 11, r. 11-B307