3109.1 Lowest Floor Height. For the new construction or substantial improvement of residential structures wholly or partially inside the flood hazard areas specified in § 3101.2(a), the lowest floor (including the basement, parking garage, and any ancillary residential uses) shall be at least the height of the DFE. The lowest floor of any underground parking garage or ancillary residential use portion of any new construction, or substantial improvement of a building located entirely outside of flood hazard areas specified in § 3101.2(a), but wholly or partially inside flood hazard areas specified in § 3101.2(b), (c), or (d), shall be at or above the DFE or be designed and constructed to be dry floodproofed during any flood up to the height of the DFE.

3109.2 Residential Enclosures. For the new construction or substantial improvement of residential structures wholly or partially inside flood hazard areas, any enclosures below the DFE shall only be used for parking of vehicles, building access, or incidental storage. The enclosure shall be designed and constructed in accordance with § 3108.8.

3109.3 Additions. For all lateral additions to an existing residential structure that constitute a substantial improvement and are located wholly or partially inside the flood hazard area, the lowest floor of the addition shall be at least the height of the DFE.

SOURCE: Final Rulemaking published at 57 DCR 10780 (November 19, 2010); as amended by Final Rulemaking published at 72 DCR 006821 (June 20, 2025).