The Zoning Regulations are silent on bay window projections into yards and courts.  The Zoning Administrator has determined that they can be treated the same as cornices and eaves, so when cantilevered, bay windows may project up to two feet (2 ft.) into any yard or court. For reference 11 DCMR B-323.2 states:   

Cornices and eaves may project over any required yard or court for a distance not to exceed two feet (2 ft.). 

Bay window projections are not included as lot occupancy nor in the measurement of Gross Floor Area (GFA) if they do not contain any walking surface or a window seat. [A window sill is OK]. For clarification, a bay window projection containing a window sill is not included as lot occupancy nor in the measurement of GFA.