D.C. Mun. Regs. tit. 11, r. 11-B310
The number of stories shall be counted at the point from which the height of the building is measured.
For the purposes of determining the maximum number of permitted stories, the term "story" shall not include cellars or penthouses or rooftop structures.
Except in the RF zones, a mezzanine shall not be considered a story in determining the maximum number of permitted stories.
In an RF zone, a mezzanine shall be considered a story in determining the maximum number of permitted stories within a principal structure, but shall not be considered a story in determining the maximum number of permitted stories within an accessory building.
Where there are multiple elevations for the finished floor of the ground floor, the height used for counting the number of stories shall be determined by the highest elevation of the finished floor.
For a building where the finished floor of the ground floor is removed or altered in height in association with a renovation where a raze of the building has not occurred, the higher of the previously existing or new finished floor of the ground floor shall be used for counting the number of stories.
D.C. Mun. Regs. tit. 11, r. 11-B310