Do you need a permit for an ADU 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.
Check your exact lot
Setbacks depend on your zoning district, and a flood zone can change the permit answer. Enter your address to see both.
Permits a ADU can need in Fairfax County
| Permit | When it applies |
|---|---|
| Building permit | An ALU is a dwelling unit and needs a building permit. |
| ALU administrative permit (interior) or BZA special permit (detached) | Interior ALUs need an administrative permit from the Zoning Administrator. Detached ALUs need a special permit from the Board of Zoning Appeals. |
| Floodplain development permit | If the lot is in a FEMA special flood hazard area, all development there needs a permit. |
Limits that apply citywide
| 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 |
R-3 Residential, 3 dwellings per acre
| 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) |
|---|
R-4 Residential, 4 dwellings per acre
| House setbacks | 30 ft front, 10 ft side, 25 ft rear, 35 ft height (cluster lots: 20 ft front, 8 ft side) |
|---|
R-5 Residential, 5 dwellings per acre
| House setbacks | 20 ft front, 8 ft side, 25 ft rear, 35 ft height |
|---|
Local details
- 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.
How to apply
Questions people ask
- When does a ADU need a building permit in Fairfax County?
- An ALU is a dwelling unit and needs a building permit.
- Do I also need a zoning permit for an ADU in Fairfax County?
- Interior ALUs need an administrative permit from the Zoning Administrator. Detached ALUs need a special permit from the Board of Zoning Appeals.
- How far from the property line can a ADU go in Fairfax County?
- 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). Setbacks vary by zoning district; check your exact lot for the district that applies.
- Where do I apply for a ADU permit in Fairfax County?
- Apply through PLUS permitting portal, or call 703-222-0801.
Sources
- 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...