D.C. Mun. Regs. tit. 11, r. 11-B323
Except for the projections and encroachments specified in this section and the exceptions in Subtitle B §324, every part of a required yard or court or other required open space shall be open and unobstructed to the sky.
Cornices and eaves may project over any required yard or court for a distance not to exceed two feet (2 ft.).
Sills, leaders, belt courses, and similar ornamental or structural features may project over any required yard or court a distance not to exceed six inches (6 in.).
The ordinary projection of skylights above the bottom of a yard or court shall be permitted if placed so as not to obstruct light and ventilation.
Awnings serving a window, porch, or door may project into a required yard or open court for a distance not to exceed forty inches (40 in.).
A chimney, smokestack, or flue may project into any required rear yard, provided the horizontal section of the projection does not exceed five square feet (5 sq. ft.).
A chimney, smokestack, or flue may project into any required side yard for a distance not to exceed two feet (2 ft.).
A self-contained air conditioner may project into any required yard or court a distance not to exceed two feet (2 ft.).
D.C. Mun. Regs. tit. 11, r. 11-B323