ADU rules in Fairfax County, VA

Yes. An ALU is a dwelling unit and needs a building permit.

Fairfax calls ADUs accessory living units (ALUs).

An interior ALU needs an administrative permit; a detached one needs a BZA special permit and at least 2 acres.

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

Limits

Interior ALU size	800 sq ft or 40% of the house's GFA, whichever is less, or the whole basement as it stood on May 10, 2023; 2 bedrooms max; inside the house with interior access
Detached ALU	2-acre minimum lot, up to 1,200 sq ft, BZA special permit
Occupancy	Owner lives in the house or the ALU; 2 people max in the ALU; permit recorded in the land records and renewed after 2 years
Parking	1 space on top of the house's requirement

House setbacks	30 ft front, 12 ft side, 25 ft rear, 35 ft height (cluster lots: 20 ft front, 8 ft side, 20 ft total)

House setbacks	30 ft front, 10 ft side, 25 ft rear, 35 ft height (cluster lots: 20 ft front, 8 ft side)

House setbacks	20 ft front, 8 ft side, 25 ft rear, 35 ft height

Fairfax is rewriting its ALU rules, and Virginia's SB 531 requires by-right ADUs from July 1, 2027, which will likely end the 2-acre minimum for detached units.
From July 1, 2027, Virginia localities must allow ADUs by right in single-family districts, with a $500 permit-fee cap and no setbacks stricter than for other accessory structures (SB 531).
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.
Lots along streams may fall in a Chesapeake Bay Resource Protection Area. Any shed, deck or addition in an RPA needs Bay Act review before a permit can issue.

Virginia USBC, 13VAC5-63-80 Section 108.2, Exemptions from application for permit. Checked 2026-10-20.
2. One story detached structures used as tool and storage sheds, playhouses, or similar uses, provided the building area does not exceed 256 square feet... 5. Fences of any height unless required for pedestrian safety as provided for by Section 3306 or used…
Fairfax County Zoning Administration, Frequently asked questions. Checked 2026-10-20.
On most residential lots, the Zoning Ordinance only allows a fence or wall of 4 feet or less in the front yard. In a side or rear yard, a fence or wall of not more than 7 feet is generally allowed... An accessory structure may NOT be located (a) in any mini…
Fairfax County Zoning Ordinance § 4102.7.B, accessory living unit. Checked 2026-10-20.
An accessory living unit must be wholly contained within the structure of a single-family detached dwelling unit... must not exceed: (a) 800 square feet of gross floor area or 40% of the gross floor area of the principal dwelling, whichever is less; or (b) …
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...