D.C. Mun. Regs. tit. 11, r. 11-413
In R-1, R-2, R-3, and R-4 Districts a retaining wall may be erected in accordance with the requirements of this section.
The height of a retaining wall shall be determined as follows:
Subject to the height limitations of § 413.4 through 413.7, the maximum height of a retaining wall shall be six feet (6 ft.).
A retaining wall shall not exceed four feet (4 ft.) in height in the following locations, unless a lower height is required by § 413.5 and 413.6:
A retaining wall located along a street frontage on a block with adjacent existing retaining walls shall not be greater in height than the tallest adjacent existing retaining walls up to the maximum height of four feet (4 ft.).
A retaining wall located on any area between a property line and a building line shall not exceed a maximum height of forty-two inches (42 in.).
A retaining wall abutting an improved alley in the R-3 or R-4 Districts shall not exceed a maximum height of twelve feet (12 ft.).
Retaining walls may be tiered or terraced provided that the width of the area between each retaining wall is at least twice the height of the lower retaining wall. The area between each wall shall be pervious and may not be paved or otherwise covered with impervious materials.
Retaining walls not meeting the requirements of this section, may be approved by the Board of Zoning Adjustment as a special exception pursuant to § 3104.1.
In addition to meeting the general conditions for being granted a special exception as set forth in that subsection, the applicant must demonstrate that conditions relating to the building, terrain, or surrounding area would to make full compliance unduly restrictive, prohibitively costly, or unreasonable.
D.C. Mun. Regs. tit. 11, r. 11-413