D.C. Mun. Regs. tit. 10, r. 10-C2305
Replacement of historic windows should be considered only if the preferred option of preserving historic windows is not feasible, given the facts and circumstances of each particular case.
Under the D.C. Construction Code (12 DCMR § 107), a permit is required for the replacement of windows in historic landmarks or buildings within an historic district.
The requirement for a permit applies equally to the removal or replacement of sash in existing frames, and to the removal or replacement of both sash and frames. If repair or rehabilitation of frames is proposed, that work shall also be indicated on the permit application.
Replacement sash and frames shall match the historic sash and frames in all respects-configuration , method of operation, profile, dimensions, material, finish, and any other salient character-defining features, except as provided below.
Panning, capping, or wrapping of window piers, mullions, frames, and sills is generally discouraged.
Replacement windows may be double-glazed, but they shall have either "true-divided" lights, or "simulated-divided" lights (integrally applied external and internal muntins), with configuration and profiles matching the historic window. False muntins or "grids" located between two panes of glass, and "snap-ins" applied either internally or externally are not considered acceptable muntins.
Glazing in replacement windows shall be clear, non-reflective glass unless otherwise historically appropriate or determined compatible by the staff. Replacement of stained or specialty glass shall match the historic glass or be historically appropriate or compatible.
Permanent exterior full screens covering the entire window opening detract from the historic appearance of windows and are discouraged. Half screens or internal screens are preferred.
The standards for window replacement reflect a hierarchy of building importance, as delineated in the following sections. The strictest standard shall be applied to National Historic Landmarks, historic landmarks and major buildings in historic districts, a more flexible standard shall be applied to contributing buildings in historic districts (with additional flexibility applied to larger buildings), and the most flexible standard shall be applied to non-contributing buildings and new construction.
If the existing windows in an historic building are not historic windows, replacement windows should be consistent with the historic window design if known, or should be consistent with the period of the building and compatible with its general historic character.
D.C. Mun. Regs. tit. 10, r. 10-C2305