D.C. Mun. Regs. tit. 10, r. 10-C2311
Windows in additions to historic buildings should be appropriate for the new addition and in most cases consistent in overall character with the windows found on the historic building. This is generally most important for windows in close proximity to a primary historic facade.
Windows in rear additions to historic buildings should generally reflect the less formal design that is characteristic at the rear of most historic structures. Alternatively, windows in an addition distinguished by deliberate contrast should be compatible in scale and character with the historic building overall.
Windows should generally be made of the same material as the building's historic windows, particularly in additions to historic landmarks, major buildings in historic districts, and small buildings in historic districts. Alternatively, use of a different window material should serve to support the design integrity of an addition distinguished by contrast.
This guidance is intended to promote design compatibility with historic buildings and districts, rather than to discourage good contemporary design or creative architectural expression.
D.C. Mun. Regs. tit. 10, r. 10-C2311