ADU Builder in Northern Virginia — Rules, Permits & Costs

Last updated August 2026. Zoning rules change — we verify the current requirements for your specific address as the first step of every ADU project.

Accessory Dwelling Units in Northern Virginia: The Complete 2026 Guide

An accessory dwelling unit — a basement apartment, an in-law suite over a garage, or a detached backyard cottage — is one of the highest-value projects a Northern Virginia homeowner can build. It adds rentable square footage, houses aging parents or adult kids, and increases resale value, all without leaving the lot you already own.

Karma Construction Group designs, engineers, permits, and builds ADUs across Fairfax County, Arlington, Alexandria, and Loudoun County under one contract. DMV permitting is one of our core competencies — and with ADUs, the permit strategy is the project strategy.

Virginia’s New Statewide ADU Law (SB 531) — What It Means for You

Virginia passed its first statewide ADU mandate in 2026. SB 531, signed in April 2026, takes effect July 1, 2027 and requires every locality to:

• Allow ADUs as an accessory use in single-family residential zones with an administrative permit — no special-use hearing
• Cap the ADU permit fee at $500
• Keep ADU setbacks no stricter than those for the primary dwelling or other accessory structures
• Drop any requirement that ADU occupants be related to the homeowner

The critical caveat: localities that already had an ADU ordinance before January 1, 2026 — Fairfax County, Arlington, Alexandria, Loudoun, and Falls Church — are grandfathered and keep their own rules, including owner-occupancy requirements. And until July 1, 2027, today’s local rules apply everywhere. That’s why the county-by-county details below matter.

ADU Rules by Jurisdiction (as of 2026)

Jurisdiction	Detached ADU?	Max Size	Owner-Occupancy	Approval Path
Fairfax County	Special permit + 2-acre lot	800 sq ft interior / 1,200 sq ft detached	Yes	Administrative (interior)
Arlington County	Yes	Full basement, or ~500–750 sq ft	Yes (with nuances)	Administrative
City of Alexandria	Yes	1/3 of main home (interior)	At permit issuance	Administrative, 10 business days
Loudoun County	Yes — by right	Up to 1,200–2,500 sq ft by district	No	By right
Prince William County	Not yet	—	—	Must allow by July 1, 2027 (SB 531)

Fairfax County ADUs (Accessory Living Units)

Fairfax County calls them Accessory Living Units (ALUs). Interior ALUs — including basement apartments — need only an administrative permit ($285 application, $75 renewal), plus building permits. Key rules: maximum 800 sq ft or 40% of the home’s floor area (or the entire basement if it existed as of May 10, 2023), maximum 2 bedrooms and 2 occupants, one additional on-site parking space, and owner-occupancy of either unit. Detached ALUs currently require a Board of Zoning Appeals special permit and a 2-acre minimum lot.

Worth knowing: Fairfax is actively studying zoning amendments (public process underway through 2026) that could ease the detached-lot minimum, basement rules, occupancy caps, and renewal requirements. If a detached ADU is your goal in Fairfax, talk to us early — the rules may be moving in your favor.

Arlington County Accessory Dwellings

Arlington allows interior, attached, and detached accessory dwellings with an administrative permit and certificate of occupancy — no public hearing and no minimum lot size. A basement ADU can occupy the entire basement; otherwise sizes scale roughly 500–750 sq ft with the home’s size. Up to 3 people may live in the AD. Parking is only required if the property has no existing parking, and Arlington charges no accessory-dwelling permit fee (building, trade, and certificate-of-occupancy fees still apply). A deed covenant is recorded with the approval.

City of Alexandria ADUs

Alexandria has allowed ADUs citywide since January 2021 with staff-level approval — Planning & Zoning completes ADU review within 10 business days of a complete application. Interior ADUs can be up to one-third the size of the main home; detached ADUs are capped by lot size (350–500 sq ft first floor for most lots) and 20 feet in height. There is no added parking requirement. The owner must occupy the property as a primary residence when the permit is issued, and ADU short-term rental is capped at 120 days per year. Homes in the Old & Historic or Parker-Gray districts also need Board of Architectural Review approval — we handle that submission as part of our permitting scope.

Loudoun County ADUs — the Most Permissive in NoVA

Under Loudoun’s 2023 zoning ordinance, accessory dwellings are by right — interior, attached, or detached — with no owner-occupancy requirement. Size caps range from 1,200 sq ft in suburban and urban districts to 2,500 sq ft in transition and rural districts. One parking space is required. On well-and-septic lots the Health Department must approve system capacity, which we coordinate as part of design. For investors and multi-generational households, Loudoun is currently the friendliest ADU jurisdiction in Northern Virginia.

Prince William County — ADUs Are Coming

Prince William County currently does not allow accessory structures to be used as dwellings in residential districts. But because the county had no ADU ordinance before January 1, 2026, it is not grandfathered under SB 531 — meaning it must allow ADUs in single-family residential zones by July 1, 2027, with a permit fee capped at $500. If you own property in PWC, now is the time to plan and design so you can file the day the door opens. (Don’t confuse this with the county’s 2025 “Affordable Dwelling Unit” ordinance — that’s a developer density program, not backyard ADUs.)

What an ADU Costs in Northern Virginia

Planning ranges for the NoVA market (contractor-published 2026 figures — your written estimate comes after a site visit):

ADU Type	Typical Range	Notes
Basement conversion	$60,000 – $145,000	~$150–$300/sq ft; least site work
Garage conversion	$80,000 – $150,000	Most garages need insulation, HVAC, plumbing upgrades
Attached addition ADU	$120,000 – $250,000	400–800 sq ft typical
Detached backyard ADU	$150,000 – $350,000+	$200–$500/sq ft by finish level

Add soft costs of roughly $8,000–$15,000 (architecture, structural engineering, permits, survey) and site work/utility connections of $10,000–$25,000 for detached units. Northern Virginia labor runs above the national average, which is exactly why value-engineering the design before permitting matters.

How We Build Your ADU — One Contract, Four Phases

1. Feasibility & design — we confirm your lot’s zoning, setbacks, and utility situation, then design an ADU that fits both your budget and your county’s rules.
2. Engineering & permits — structural drawings, the ADU/ALU application, building permits, and any health-department or architectural-review approvals. Permitting is a core competency, not an afterthought.
3. Construction — one project manager, our own crews, a published schedule.
4. Certificate of occupancy — final inspections, recorded covenants where required, and keys in hand.

Considering a factory-built unit instead? See our modular ADU options in Virginia — often faster and more cost-predictable for detached units. ADUs also pair naturally with a home addition or a whole-home remodel when you’re reworking the property anyway.

Why Northern Virginia Homeowners Are Building ADUs Now

• Rental income in one of America’s strongest rental markets
• Multi-generational living — aging parents close, but independent
• Property value — finished, permitted square footage appraises
• A friendlier legal landscape every year — SB 531 statewide, and Fairfax’s rules under active review

Ready to find out what’s possible on your lot? Call 202.798.7663 or request a free ADU feasibility consultation below — we’ll give you a straight answer on zoning, timeline, and budget.