Can You Build an ADU or In-Law Suite in Northern Virginia? Fairfax, Arlington and the New Virginia ADU Law Explained (2026)

Key takeaways

• In Fairfax County the legal term is Accessory Living Unit (ALU). "ADU" on county housing pages usually means the Affordable Dwelling Unit program.
• Fairfax today: one interior ALU per single-family detached home, up to 800 sq ft or 40% of the house, or the whole basement; owner on site; one extra parking space.
• Fairfax is proposing detached ALUs by right on lots of 36,000 sq ft or more, no separate zoning permit, no owner-occupancy rule and a 1,200 sq ft cap. Hearings: Oct 29 and Nov 17, 2026.
• Arlington allows basement, attached and detached units. The owner must live on the property.
• Virginia SB 531 takes effect July 1, 2027, but exempts localities with an ADU ordinance adopted before 2026. Fairfax staff say it does not apply to Fairfax.
• Realistic paths: basement suite, garage conversion, attached addition, or (on the right lot) a detached unit.

What is an ADU, and what is it called in Fairfax and Arlington?

An accessory dwelling unit is a second, smaller home on a single-family lot with its own space to sleep, cook, eat and bathe. Families build them for aging parents, adult children, caregivers or rental income. The vocabulary in Northern Virginia matters when you search county pages or talk to a permit reviewer:

• Fairfax County: Accessory Living Unit (ALU), a secondary unit subordinate to a single-family detached home, "sometimes referred to as an in-law suite."
• Arlington County: Accessory Dwelling (AD), a complete dwelling with kitchen and bath on a lot with a main dwelling under the same ownership.
• Virginia, from July 1, 2027: accessory dwelling unit (ADU), an attached or detached unit with complete living facilities, including cooking and sanitation.

Fairfax vs. Arlington vs. the new Virginia law: how do the rules compare?

This summary reflects the rules as of October 20, 2026. Fairfax has two columns because the county is rewriting its ALU rules, and the changes are only proposals until the Board of Supervisors votes.

These are zoning rules only. Building codes, Health Department rules for wells and septic systems, stormwater and tree rules, and HOA covenants still apply on top.

What are Fairfax County's current ALU rules?

Under Zoning Ordinance subsection 4102.7.B, as summarized on the county's ALU permit page, beyond the limits in the table:

• The unit must connect to the house through a finished, heated and cooled interior space.
• Any new outside entrance must be on the side or rear of the house.
• Garage space does not count toward the size limit.
• The parking plat must show three spaces on a public street, or four on a private street.
• The administrative permit is filed in the county's PLUS portal (about 30 days to process), recorded in the land records and renewed after an initial two years.
• Detached units, or interior units that break a standard, need a Board of Zoning Appeals special permit.

What is Fairfax County proposing to change?

The proposed amendment, with a staff report and an FAQ dated October 2, 2026, goes to the Planning Commission on October 29, 2026 at 7:30 p.m. and the Board of Supervisors on November 17, 2026 at 4:30 p.m. Staff recommend:

• No separate zoning permit for by-right ALUs; building permits still apply.
• Detached ALUs by right on lots of 36,000 sq ft or more, by special permit on smaller lots. The Board may raise or lower that threshold.
• Size: 1,200 sq ft or 40% of the house, whichever is less, or the whole basement.
• No owner-occupancy rule, three occupants instead of two, and no extra parking space.
• A unit linked by a hallway or breezeway of 10 feet or less counts as interior ("semi-detached").

Short-term lodging such as Airbnb would still be prohibited on a lot with an ALU. Staff recommend the change take effect the day after adoption.

What are Arlington County's accessory dwelling rules?

Arlington has allowed accessory dwellings since 2009 and new detached ones since July 1, 2019. Key points from the county's Accessory Dwelling Permit page:

• Eligible lots: a one-family detached house in the R-20, R-10, R-8, R-6, R-5, R-10T, R-15-30T or R-2-7 districts.
• Owner occupancy: the owner must live in the main house or the AD; otherwise the whole lot is limited to one family. The AD holds up to three people.
• New detached buildings: height up to 1.5 stories or 25 feet, at least 25 feet from a street, 8 feet from the house and 5 feet from side and rear lot lines on interior lots.
• Process: AD Permit in Permit Arlington (about 30 days), recorded Declaration of Covenants, building permit, Affidavit of Compliance and Certificate of Occupancy.

Does the new Virginia ADU law (SB 531) apply in Fairfax and Arlington?

SB 531, approved April 13, 2026 (Chapter 895), is codified as § 15.2-2292.4 and takes effect July 1, 2027. Where it applies, ADUs become a permitted accessory use in single-family districts, the locality's ADU permit fee is capped by statute, and the permit must be issued if the unit complies. Localities may still require 30-day minimum leases, parking, owner occupancy at the time of application, and a size cap of no less than 500 sq ft. They may not require restrictive covenants or that occupants be related.

Two limits matter here. The law does not override HOA covenants or condominium instruments. And it does not apply to a locality that adopted an ADU ordinance before January 1, 2026. The Fairfax staff report states the law does not apply to Fairfax County. Arlington's rules date from 2008, so the exemption appears to cover Arlington too, although we found no county statement confirming it. In both counties, follow the county's own rules. One caution: the bill as introduced would have barred most parking requirements and capped setbacks at five feet, but those limits were dropped before passage.

Which type of ADU fits your home?

The rules decide what is allowed. Your house, lot and budget decide what makes sense. Timelines below come from Dzala's service pages; for cost, we quote each project after a free site visit.

*Timelines for basement, garage and addition work are taken from Dzala's own service pages and cover construction; county review time is extra. For pricing, contact Dzala at (703) 919-1645 or dzalausa@gmail.com for a free, project-specific estimate.

How do you get an ADU approved in Fairfax or Arlington?

1. Confirm who regulates your address. Properties inside the Town of Vienna or the City of Falls Church follow those localities' rules; nearby addresses outside their limits fall under Fairfax County.
2. Check zoning and house type. Both counties allow accessory units only with a single-family detached house.
3. Read your HOA covenants. The county will not enforce them, but they can still restrict a second unit.
4. Test your plan against the limits. Check floor area, entrances, parking, setbacks and egress before design.
5. Talk to the county early. Arlington recommends a pre-submission meeting with its Zoning Division; Fairfax's Zoning Permits Section answers ALU questions.
6. Prepare drawings. Expect floor plans, a plat or site plan with parking, and photos of existing space.
7. Apply for zoning approval. Fairfax: ALU administrative permit in PLUS, or a special permit for detached units. Arlington: AD Permit, then record the Declaration of Covenants.
8. Pull building permits and build. Trade permits and inspections follow; wells and septic systems need Health Department review.
9. Close out before move-in. Fairfax currently requires the permit to be recorded first. Arlington requires an Affidavit of Compliance and a Certificate of Occupancy.

What does an in-law suite cost in Northern Virginia?

There is no honest single number, and we would rather not publish one. Cost depends on whether you finish existing space or build new, how far plumbing must run (below-grade plumbing is usually the biggest driver in a basement), whether you need a full kitchen, and the finish level.

• Basement suite: every basement is different. Contact Dzala at (703) 919-1645 or dzalausa@gmail.com for a free, project-specific estimate.
• Garage conversion: the price depends on your slab, insulation and door opening. Call (703) 919-1645 and we will walk the space and quote it.
• Attached addition: size, foundation and roof tie-in drive the number. Ask us for an estimate based on your lot and plans.
• Detached unit: utilities, site work and permits vary widely from lot to lot. Email dzalausa@gmail.com for a free site review and estimate.

County permit and building fees apply on top of construction; your county's current fee schedule lists them. Fairfax also notes that any improvement, including an ALU, may raise a property's assessed value.

Frequently asked questions

Planning an in-law suite? Start with the lot, not the floor plan

Dzala General Contractor builds basement suites, garage conversions and additions in Tysons, McLean, Vienna, Oakton, Great Falls, Arlington, Alexandria and Falls Church, plus Bethesda and Potomac. We confirm which jurisdiction governs your address, check the limits and handle the permits. Book a site visit or call (703) 919-1645.

General information as of October 20, 2026, not legal advice. The Fairfax amendment described here was proposed, not adopted, at the time of writing. Confirm requirements with your county's zoning office.

Sources

• Virginia LIS: SB 531, 2026 Regular Session (bill history)
• Virginia Acts of Assembly 2026, Chapter 895 (SB 531, approved April 13, 2026)
• Code of Virginia § 15.2-2292.4 (effective July 1, 2027)
• Fairfax County: Accessory Living Unit (Administrative Permit) and ZO 4102.7.B standards
• Fairfax County: Accessory Living Units, Proposed Zoning Ordinance Amendment (hearing dates)
• Fairfax County: Accessory Living Unit FAQs (October 2, 2026)
• Fairfax County: ALU amendment staff report (PLUS document)
• Fairfax County: Affordable Dwelling Unit (ADU) and WDU program resources
• Arlington County: Accessory Dwelling Permit
• Arlington County: Accessory Dwelling Regulations Update (2017 and 2019 amendments)