D.C. Mun. Regs. tit. 11-K, § 225
225.1 A rear yard of two and one-half inches (2.5 in.) per one foot (1 ft.) of height or a minimum of twelve feet (12 ft.) shall be provided for each structure located in the SEFC-3 zone.
SOURCE: Final Rulemaking published at 63 DCR 2447, 3176 (March 4, 2016 – Part 2).