ADU Builder in Fairfax, VA

A Fairfax address can mean two different governments. The City of Fairfax is an independent city with its own zoning ordinance and its own permit office, and in July 2026 its council voted to allow detached ADUs by right. Many homes with a Fairfax mailing address sit outside the city in unincorporated Fairfax County, where a backyard unit still takes two acres and a public hearing. DLA Design & Build starts every Fairfax ADU project by finding out which rules apply to your lot, then designs a unit that fits them and handles the permits.
One zip code, two rulebooks. We start by finding out which one is yours.

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Why Fairfax Homeowners Want a Second Unit

Inside the city, the neighborhoods are mostly postwar: ramblers, split levels and Cape Cods in Mosby Woods, Country Club Hills and the streets around Old Town, many on lots with a real backyard. George Mason University sits just south of the city line, and faculty, staff and graduate students all need places to live close in. A small, well-built accessory dwelling unit behind a city house fills that need and helps the owner with the mortgage.
Family is the other big driver, same as everywhere in Northern Virginia. A parent moves closer, or an adult child comes home and needs some privacy. On the county side, in areas like Fair Lakes and Mantua, that usually means a lower-level apartment or an attached suite, because the county's detached rules rule out most lots there.
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ADU Types We Build in Fairfax, VA

Each section below covers the City of Fairfax rules first, then the Fairfax County rules that apply to Fairfax addresses outside the city.

01Detached ADU

Detached ADUs in the City of Fairfax

Unlike most of Fairfax County, the City of Fairfax now allows a backyard cottage by right. Since the council's July 2026 vote, a single-family or duplex lot in the city can have one detached ADU without a public hearing. It cannot sit between the house and the street, and it has to be at least 20 feet from the rear lot line, with side setbacks of 8 or 12 feet depending on the zoning district, and no taller than 20 feet or the house, whichever is lower. Its floor area cannot exceed 35 percent of the house.

Outside the city, Fairfax County still requires two acres and a Board of Zoning Appeals special permit, so a county lot near Fair Lakes rarely qualifies.

02Attached ADU

Attached ADUs in Fairfax

An attached ADU shares a wall with the house and has its own kitchen, bath and door. In the city, a new exterior entrance for an attached unit goes on the side or rear, and the unit is held to the same 35 percent size limit and two-bedroom cap as a detached one. In the county, an attached unit is approved administratively as an interior ALU if it connects to the main house through finished, conditioned interior space and stays within the lesser of 800 square feet or 40 percent of the house.

On a Mosby Woods rambler, a side wing with a private entrance often lays out better than carving the unit out of existing rooms.

03Basement ADU

Basement ADUs in Fairfax

A lower-level apartment is the option that works almost everywhere around Fairfax. In the city, a basement unit counts against the same 35 percent limit as any other ADU, and the owner still has to live in one of the two homes. On the county side, an existing basement can be used in full under a date cutoff in the ordinance, which can make a big finished basement in Fair Lakes or Mantua the easiest way to add a unit.

The construction is the same on both sides of the line: bedroom egress windows, a side or rear entrance, moisture control and code-compliant ceiling height. We run it like any basement remodeling project, with the added systems a second home needs.

04Garage Conversion

Garage Conversions in Fairfax

Many older Fairfax houses have a one-car garage that holds more storage than cars. Converting it into a unit is allowed in the city as an attached ADU, but the lot still needs at least two off-street driveway spaces once the garage is gone. That usually means the driveway has to be long or wide enough for two cars. In the county, a converted garage stays inside the house as an interior ALU, and the parking standard is the house's requirement plus one space.

We level and insulate the floor, frame the old door opening with windows that fit the house, and bring the space up to living code.

05In-Law Suite

In-Law Suites in Fairfax

Not every in-law suite has to be a permitted ADU. The city describes an ADU as a unit with its own kitchen, sleeping and bathroom facilities, so a bedroom, bath and sitting room that share the family kitchen stay part of the house. The county draws the same line for its ALUs and uses a second kitchen or wet bar certification when a building permit shows extra kitchen facilities that are not meant to be a separate home.

The permitted route makes sense when a parent wants full independence or you may rent the space later. Either way, we design a first-floor or walkout suite with a no-step entry, wide doors and a curbless shower.

06Guest House

Guest Houses and Studios in Fairfax

In the city, a detached building with its own kitchen, sleeping area and bath is a detached ADU, so it follows the one-per-lot limit, the 35 percent cap, the 20-foot height limit and the owner-occupancy rule. A studio, home office or pool house without living quarters is an ordinary accessory structure, and we confirm the city's accessory structure rules for your zoning district before design.

In the county, a detached building set up as a separate home with its own kitchen is a detached ALU, which means two acres and a special permit. If you might rent the building someday, build it as a legal city ADU from the start.

07Above-Garage ADU

Garage Apartments in Fairfax

A carriage-house unit over a detached garage is now possible in the city as a detached ADU, but the height rule is tight. A detached ADU cannot be taller than 20 feet or the height of the house, whichever is less, and on a one-story Fairfax rambler, the house height can be the lower of the two. Fitting a garage and a living level under that line takes careful roof design, often with dormers.

Over an attached garage, the unit counts as attached, with its entrance on the side or rear. On the county side, a unit over a detached garage needs two acres and a special permit. Our garage additions work covers new garages framed to carry a floor above.

08ADU Addition

ADU Additions in Fairfax

Building a new wing for the unit is how you get a comfortable layout on a small city rambler. The addition follows the city's regular setbacks for your zoning district, and the unit inside it is held to 35 percent of the house's gross floor area. On the county side, an ADU addition qualifies as an interior ALU when it stays attached through finished interior space and within the county's size cap.

Permits go to one government or the other: the City of Fairfax for city lots, Fairfax County Land Development Services for county lots. We plot the setbacks first so the wing lands where the reviewers will approve it.

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City of Fairfax vs. Fairfax County ADU Rules

In the City of Fairfax, ADU standards sit in the city's own zoning ordinance. After the July 28, 2026 vote, a single-family or duplex lot can have one ADU, attached or detached, without a public hearing. The ADU's gross floor area can't exceed 35 percent of the house, it can have up to two bedrooms, and the owner has to live in either the house or the ADU. The lot needs at least two off-street driveway spaces. A detached unit has to stay out of the space between the house and the street and sit at least 20 feet from the rear lot line. Side setbacks are 8 or 12 feet depending on the zoning district. The old age and disability occupancy limits were dropped. Construction needs both a building permit and a zoning permit from the city.
Outside the city, unincorporated Fairfax County's accessory living unit rules apply. An interior unit, in a basement or an attached addition, can go through an administrative permit if it meets the county's size, entrance and parking standards. A detached unit needs a Board of Zoning Appeals special permit and a lot of at least two acres, which few county lots around Fairfax have. The county is working on changes to those rules, but none had been adopted as of September 2026. County building permits come from Land Development Services.

If you're weighing an ADU against a simpler remodel, our Fairfax remodeling team can compare both approaches for the same space.
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Frequently Asked Questions About ADUs in Fairfax, VA

Don't go by the mailing address. Fairfax addresses cover both the independent city and a large part of unincorporated Fairfax County. Look at who sends your real estate tax bill: a city bill means city rules, and a county bill means county rules. We also check the parcel on the city and county property records before starting a design, because the answer decides whether a backyard cottage is even on the table.

Probably, if the numbers work. Since the council's July 2026 vote, the city allows a detached ADU by right on single-family and duplex lots. The unit can't sit between the house and the street, has to be at least 20 feet from the rear lot line, and no taller than 20 feet or the height of your house, whichever is lower. Side setbacks depend on your zoning district. On a typical city lot, the 35 percent size limit and the rear setback usually decide the footprint. The detached ADU section on this page covers siting in more detail.

The city caps an ADU's gross floor area at 35 percent of the house's gross floor area, whether it's attached or detached. It's a percentage, not a fixed number, so a larger house allows a larger unit. The unit can have up to two bedrooms. On a smaller rambler, 35 percent doesn't leave much room, so a one-bedroom plan with a combined living and kitchen area often works better than squeezing in a second bedroom. We measure your house's gross floor area early so you know the real limit before anything is drawn.

No. The city's 2026 change made attached and detached ADUs allowed by right, so a unit that meets the standards doesn't go to a hearing. You do need a zoning permit and a building permit. The city's Code Administration reviews the plans against the building code, and the Zoning Division checks them against the zoning ordinance. The building permit ends with a certificate of occupancy once the final inspection passes.

On a typical lot, an interior unit. Fairfax County allows an accessory living unit inside the house by administrative permit, capped at the lesser of 800 square feet or 40 percent of the house, with a pre-existing basement allowed in full under the county's cutoff date. It can have two bedrooms and two occupants, the owner lives on site, and you need one parking space beyond what the house already requires. Detached units need two acres and a special permit, which rules out most lots there.

Probably not soon. Virginia Code section 15.2-2292.4, effective July 1, 2027, requires localities to allow ADUs in single-family districts. But it says it doesn't apply to a locality that adopted an ADU ordinance before January 1, 2026. The City of Fairfax and Fairfax County both had ADU rules before that date. For now, each keeps its own standards. We design to whichever one covers your lot and watch for changes from either government.

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How We Handle a Fairfax ADU

Step one is the jurisdiction check. After that, we measure the house, confirm setbacks and parking, and look at utilities: whether water, sewer and electric can serve a second unit from the existing lines or need a new service.
For a city project, we submit the zoning and building permits to the City of Fairfax. For a county project, it's the ALU permit plus the Land Development Services building permit. Either way, you get one point of contact from design through the certificate of occupancy.

The Result

A permitted second unit that fits the lot and the neighborhood and holds up when a buyer's lender or appraiser starts asking questions years from now.
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ADU Builder in Nearby Communities

We plan and build ADUs across Northern Virginia. See local details for nearby areas.

Plan an ADU in Fairfax

Call (703) 743-7777 or fill out the form with your address.
We'll tell you whether the city or the county governs your lot and what that means for the unit you have in mind.

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