D.C. Mun. Regs. tit. 11, r. 11-107
The zone district boundaries shall be shown on each section of the Zoning Map.
The scale of the Zoning Map and the dimensions entered on the map shall be shown on each section of the map to serve as guides.
Dimensioned district boundaries showing on the Zoning Map are intended to coincide generally with lot lines. Where a dimensional boundary line coincides within one foot (1 ft.) or less with a lot line of a lot of record on May 12, 1958, that boundary line shall be construed to be the lot line at that location.
Whenever a portion of any district is indicated as a strip paralleling an opened or unopened street, the width of this strip, unless delimited by lot lines or otherwise dimensioned, shall be assumed to be one hundred feet (100 ft.) measured at a right angle from the nearest street to which it is parallel and adjacent.
In all other cases, the zone district boundary lines shall be intended to follow existing lot lines, the center lines of streets, alleys (including any closed streets or alleys not previously zoned), and natural water courses.
In the case of tidal water areas, the zone district boundary shall be either the mean high water level or the established pierhead lines, whichever gives the greatest control.
In cases of disagreement or uncertainty existing as to the exact location of a zone district boundary line, the Board of Zoning Adjustment, upon appeal filed in accordance with §§ 3100.2 and 3112, shall determine the exact location of the boundary.
When a zone district boundary line divides a lot that was in single ownership on May 12, 1958, the permitted use, height, density, or bulk of a structure located on that lot may be determined by § 2514.
D.C. Mun. Regs. tit. 11, r. 11-107