D.C. Mun. Regs. tit. 14 § 502
Each habitable room shall have a glass area transmitting natural light equivalent to that which would be transmitted by a clear glass area at least equal to one tenth (1/10) of the floor area served, consisting of one or more of the following:
Rooms opening on enclosed porches and meeting the lighting requirements of Article 501-01-e of the 1941 Building Code, as amended (Title 12, DCMR) and rooms lighted through sunporches and meeting the lighting requirements of § 3 -515 of the 1961 Building Code, as amended, shall be deemed to have adequate natural light.
The sash area of openable windows, side lights, or transoms, the horizontal projection of the glass area of skylights, and in all other instances the gross glass area, shall be used in computing the required glass area.
Any portion of any glass area facing directly on any wall, portion of a structure, or other light obstruction less than three feet (3 ft.) from that glass area, shall not be included as contributing to the required natural light.
At least fifty percent (50%) of the required glass area shall be a window, glazed door, side light, or transom, each glazed with clear glass.
Obscure glass, glass blocks, or other approved translucent material may be used to transmit up to fifty percent (50%) of the required natural light.
D.C. Mun. Regs. Tit. 14, § 502