D.C. Mun. Regs. tit. 11, § 2812
2812.1 Wherever preferred uses, as defined in §2811.1, are required or allowed pursuant to §§ 2811.2 and 2811.3, such ground floor preferred uses shall:
(a) If located on a corner, wrap around the corner to a minimum depth of 20 feet on the side street;
(b) Occupy the ground floor to a minimum depth of 30 feet;
(c) Have a minimum clear floor-to-ceiling height of fourteen (14) feet, measured from the finished grade, for the area of the ground floor dedicated to preferred uses;
(d) The street-facing facades of buildings on primary streets shall devote not less than seventy-five (75) percent per individual use or fifty (50) percent of the length and fifty (50) percent of the surface area of the street wall at the ground level to windows associated with preferred uses or windows associated with main building entrances; and
(e) The street-facing facades of mixed-use or non-residential buildings on secondary streets shall devote not less than seventy-five (75) percent per individual use or thirty (30) percent of the length and thirty (30) percent of the surface area of the street wall at the ground level to windows associated with preferred uses or windows associated with main building entrances.
2812.2 The windows required by § 2812.1(d) shall have clear or clear/low emissivity glass allowing transparency to a depth of twenty (20) feet into the preferred ground level space with bottom sills no more than four (4) feet above the adjacent sidewalk grade.
2812.3 Such windows must allow views from within the building to the street.