Can you build an ADU in Fairfax County?
Right now Fairfax only lets you put a second living space inside your existing house. A separate backyard cottage needs a special permit and a lot of at least two acres. That changes on July 1, 2027, when state law requires Fairfax to allow detached units as a permitted accessory use. If your property is in an HOA, the state law does not override your covenants.
What’s true today
Can you build one?
Allowed, with conditions. Fairfax calls these accessory living units, not ADUs — locally "ADU" means an Affordable Dwelling Unit under a different program entirely. An accessory living unit is allowed only alongside a single-family detached house, one per lot, and by administrative permit it has to sit wholly inside the existing house with a finished, temperature-controlled interior connection to it. A freestanding unit is possible only by special permit from the Board of Zoning Appeals, and only on a lot of at least two acres. Any new outside entrance has to be on the side or rear.
§ 4102.7.B(1)–(3), (15)(a)Read the sectionRead at source Sep 6, 2026How big can it be — Inside your existing house?
Up to 800 sq ft, or 40% of your house’s floor area — whichever is smaller. Whichever is smaller. There is an alternative for basements: you may use the whole basement or cellar, up to its size as of 10 May 2023. Gross floor area here counts basement space with at least 6 ft 6 in of headroom, but not a garage. No more than two bedrooms either way.
§ 4102.7.B(4), (5)Read the sectionHow big can it be — A separate building in the backyard?
Up to 1,200 sq ft. Applies only by special permit, and only on lots of at least two acres. It must also stay clearly subordinate to the house. This route is not available by administrative permit at all.
§ 4102.7.B(15)(a), (16)Read the sectionHow far from the property line?
Not addressed in § 4102.7.B. Setbacks come from the general accessory-structure rules elsewhere in the ordinance and still need reading at source. From 1 July 2027 the state caps rear and side setbacks at whichever is less: the setback required for the house, or the one required for accessory structures on the lot.
Do you have to live there?
Yes. Either the accessory living unit or the main house has to be owner-occupied. The unit itself may house no more than two people, and the ordinance also limits who may occupy the main house. From 1 July 2027 the state prohibits localities from requiring a family relationship between the occupants of the two units.
§ 4102.7.B(6)–(8)Read the sectionRead at source Sep 6, 2026Do you need more parking?
1 extra space. A house with an accessory living unit needs the parking normally required for the house plus one more space, and one space on the lot must have convenient access to the street. The Board of Zoning Appeals can waive this by special permit, and can also require more. What the state law does to this from 1 July 2027 still needs checking against § 15.2-2292.4 directly — the bill summary and the codified text read differently on parking, so do not state it either way yet.
§ 4102.7.B(9), (15)(d), (17)Read the sectionRead at source Sep 6, 2026What does the permit cost?
$285. Administrative permit $285, renewal $75, per the county’s accessory living unit page. Building permit and plan review fees are separate and not included here. From 1 July 2027 the state caps the ADU permit fee itself at $500.
Read the sectionRead at source Sep 6, 2026Can you rent it out?
Not addressed for accessory living units in § 4102.7.B. From 1 July 2027 the state allows a locality to require a minimum lease of 30 consecutive days, which would rule out short-term rental. Whether Fairfax takes that option is not yet decided.
Which kinds are allowed?
Because the unit must sit wholly within the existing house, basement and interior conversions are the realistic routes today. A detached new build needs a special permit and at least two acres. A detached garage conversion is not permitted by administrative permit; whether an attached garage conversion qualifies is not spelled out in § 4102.7.B and needs a call to Zoning.
§ 4102.7.B(2), (15)(a)Read the sectionRead at source Sep 6, 2026The rules are being rewritten right now
Fairfax is actively rewriting its accessory living unit standards. Draft text exists and community meetings ran through January, April and July 2026, with Land Use Policy Committee and Planning Commission sessions in December 2025 and July 2026. No public hearing has been scheduled. Notably the county’s amendment page does not mention SB 531 or the 2027 state law as a driver. State law expressly lets a locality adopt a substantially compliant ADU ordinance before 1 July 2027, so Fairfax could land somewhere ahead of the deadline — recheck this page before publishing and at every quarterly pass.
Read the sectionRead at source Sep 6, 2026If you’re in an HOA
The state law does not override your HOA. Check your deed and closing paperwork for a recorded declaration of covenants, or look the property up in the Fairfax County land records. If there is an association, its governing documents decide this, not the county and not the state law.
Va. Code § 15.2-2292.4(F)Read the sectionRead at source Sep 6, 2026What the process looks like
Administrative permit application to the Zoning Administrator. Health Department sign-off first if the property is on a well or septic system. The unit has to meet building, safety, health and sanitation rules, and needs an interconnected smoke and carbon monoxide alarm and a fire extinguisher. Before anyone moves in, the owner records a copy of the permit in the county land records, indexed under the owner’s name. The permit runs two years initially, then renews for up to five years at a time on a clean compliance record. Building a unit does not subdivide the lot.
§ 4102.7.B(10)–(13)Read the sectionRead at source Sep 6, 2026Every rule on this page links to the section of the Fairfax County ordinance it comes from, so you can read it yourself. Localities are amending these rules ahead of July 1, 2027 — worth confirming the section before you commit money to a design. This is general information about Fairfax County, not a determination about your particular lot.
What does this mean for your lot?
Three questions. We’ll check the Fairfax County rules against your situation and tell you what you’re actually looking at — including if the answer is no.