The height of a retaining wall shall be determined as follows:
Subject to the height limitations of Subtitle C § 1401.3 through 1401.6, 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 Subtitle C §§ 1401.5 and 1401.6:
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 RF zones 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.
D.C. Mun. Regs. tit. 11, r. 11-C1401