D.C. Mun. Regs. tit. 14, r. 14-504
Each bathroom shall be naturally or artificially lighted at least equivalent to the requirements of this section.
A naturally lighted bathroom shall have a glass area consisting of one or more of the following:
The glass area shall face directly to the outside, and shall transmit 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.
No glass area shall contain less than four square feet (4 ft.2).
Obscure glass, glass blocks, or other approved translucent material may be used to transmit up to fifty percent (50%) of the required natural light.
The sash area of openable windows 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.
Artificially lighted bathrooms shall be illuminated with a minimum of ten (10) foot candles measured at floor level at any point on the open floor space.
D.C. Mun. Regs. tit. 14, r. 14-504