ADU rules in Washington, DC

Yes. A new unit needs a building permit and a certificate of occupancy.

In R zones a second unit is an accessory apartment: one per lot, up to 35% of the house, 3 occupants max.

In RF zones a basement unit is the second unit of a flat, allowed by right.

Rules last checked by a person on October 20, 2026. Guidance only; confirm with Washington before you build.

Limits

Units per lot	1 accessory apartment (R zones); 2 principal units in a flat (RF zones)
Size inside the house	Up to 35% of the house's gross floor area (R zones)
Minimum house size	2,000 sq ft in R-1; 1,200 sq ft in R-2 and R-3
Detached building	650 sq ft footprint in R-1/R-2, 550 sq ft in R-3/RF; 22 ft and 2 stories; 3 ft side and 5 ft rear from neighboring lots (R zones)Confidence: medium
Occupancy	3 people max in the accessory apartment; owner lives in the house or the apartment
Entrance	No new entrance on a street-facing wall

Renting the unit needs a residential rental business license and a housing inspection first.
Fire separation, egress windows and ceiling height drive most DC basement-unit designs. Plan for them before you draw the layout.
Call 811 at least a few days before you dig footings or post holes. Utility locating is free.
HOA covenants and deed restrictions can be stricter than city rules. Check yours before you build.

DC Department of Buildings, How to get a permit. Checked 2026-10-20.
Unless you are in a historic district, the following work does not require a building permit: ... Single garden storage shed that does not exceed 50 square feet (4.65 m2) in area and is less than 10 feet (3048 mm) in overall height ... Repair of existing fe…
DC Zoning Regulations, accessory buildings (11-D §§ 5000-5005, 11-E §§ 5000-5005), as amended by ZC Case 25-12. Checked 2026-10-20.
Accessory buildings: 22 ft and 2 stories; footprint the greater of 30% of the required rear yard or 650 sq ft (R-1, R-2) / 550 sq ft (R-3, RF); not in the required rear yard except a shed; 7.5 ft from the centerline of any alley; in R zones 3 ft from a side…
44 CFR 59.1, definition of development (NFIP). Checked 2026-10-19.
Development means any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures...
44 CFR 60.3, floodplain management criteria. Checked 2026-10-19.
Require permits for all proposed construction or other development in the community...