Building an accessory living unit (ADU) in Fairfax County

Where these rules apply

This page covers the rules in the Fairfax County Zoning Ordinance. The county completely surrounds the City of Fairfax and is next to the cities of Falls Church and Alexandria. In Virginia, cities and counties are separate governments that don't overlap, so those cities set their own rules.

Three incorporated towns, Clifton, Herndon and Vienna, sit inside the county. The county's building permit guide says projects in the towns of Clifton or Vienna "require separate approvals from the respective towns," and the county's zoning code list shows Herndon and Vienna with their own zoning districts. If you live in a town, confirm with the town which ALU rules apply before you design.

For Virginia's statewide rules, see our Virginia page. [4][5][6]

What you can build today

The rules are in subsection 4102.7.B of the Zoning Ordinance. An ALU is allowed only with a single-family detached house, and only one per lot. It needs areas for eating, sleeping, living and sanitation.

Interior ALU, by administrative permit. The ALU must be "wholly contained" inside the house, with direct access to the main home through an interior space that is finished, temperature-controlled and fully enclosed. A basement apartment or an addition can qualify. Any new outside entrance must be on the side or rear of the house. A new garage or carport must sit next to the existing one and use the same driveway and curb cut.

Special permit. The Board of Zoning Appeals (BZA) can approve a special permit, after a public hearing, to:

• put the ALU in a freestanding building, but only on a lot of at least two acres;
• move the outside entrance or change access from the street;
• exceed the interior size limit; or
• waive the extra parking space.

Limits for every ALU. No more than two bedrooms and no more than two people in the ALU. Either the ALU or the main house must be owner-occupied. The ALU must have a working fire extinguisher, and smoke and carbon monoxide detectors (when required for a fireplace or gas service) interconnected with the main house. [1][2][7]

Size, height and setbacks

Interior ALU. The ALU can be up to 800 sq ft of gross floor area or 40% of the main house, whichever is less. Or it can take up the entire basement or cellar, at the size it had on the date the ordinance sets. The ordinance text gives that date as May 10, 2023; the county's administrative permit page still shows July 1, 2021, so confirm with the Zoning Permits Section if your basement was changed between those dates. Basement space with 6 ft 6 in of headroom or more counts toward the size, but a garage doesn't. The BZA can approve a larger interior ALU by special permit.

Detached ALU. A detached ALU approved by special permit can have up to 1,200 sq ft of gross floor area and "must be clearly subordinate to the principal dwelling."

Height and setbacks for detached units. The county's 2026 staff report describes the general rules for freestanding accessory buildings. On lots of 36,000 sq ft or less, they are limited to 20 ft in height. On larger lots, which includes every lot that can have a detached ALU today (at least two acres), there is no specific height limit, but the building must be subordinate to the house. The report's comparison table lists these setbacks for a detached ALU under the current rules: 5 ft from the side and rear lot lines if it is 8.5 to 12 ft tall; if it is taller than 12 ft, a distance from the rear lot line equal to its height, and outside the district's side setback (for example, 20 ft in the R-C to R-1 districts). The BZA can also add conditions when it approves a special permit. Confirm the rules for your lot with the Zoning Permits Section. [1][2][7]

Parking

A house with an ALU must have the on-site parking spaces required for the house plus one more. Only one space serving the lot needs convenient access to the street.

The county's application instructions put it in practical terms: your plat must show a total of three parking spaces if the house is on a public street, or four on a private street. The spaces must stay available for parking, so a garage you count can't be used for storage.

If you can't provide the extra space, you can ask the BZA to waive it through a special permit. The BZA can also require more parking than the minimum. [1][2]

Fees and timeline

The administrative permit application fee is $285, and a renewal is $75. You pay electronically at the end of the application in the county's PLUS portal; a 2.35% service charge is added for credit cards. A special permit for an accessory use is $615 under the county's zoning fee schedule. Building, electrical, plumbing and other construction permits carry their own fees through Land Development Services.

The county says an administrative permit "may take approximately 30 calendar days to process." If you need a building permit to create the ALU, the county reviews the administrative permit and then holds it until the building permit passes final inspection.

The administrative permit is issued for an initial two years. The Zoning Administrator can extend it for up to five years at a time, based on your record of compliance. A special permit is also approved for two years at first and can be extended for up to five years at a time. [1][2][7]

Building permits and well or septic

The zoning permit doesn't replace construction permits. The county may also require building permits for new work or to bring existing space into compliance, such as a permit for a stove, an emergency exit from a bedroom, or electrical work done without a permit.

Residential additions and alterations, including finished basements, are filed online in PLUS under the Building tab. Property owners may obtain permits in their own name, but the county strongly recommends that a licensed contractor pull them. Electrical and plumbing permits can be obtained after the building permit is issued.

If the house is on a well or septic system, you must get Health Department approval before the ALU permit is approved. The Health Department may require more documentation for added bedrooms. [1][2][5]

Renting it out and short-term rentals

The owner must live on the property, in either the ALU or the main house. The ALU is limited to two occupants. The main house is limited to a family, one or two people with their dependent children, or up to four unrelated people living as one household.

Short-term lodging, such as Airbnb or VRBO, needs its own zoning permit, and the county's 2026 staff report says "The Zoning Ordinance prohibits short-term lodging on a lot with an ALU."

The ordinance says building an ALU is not a subdivision of the lot it sits on. [1][7]

Proposed changes for late 2026

The county has drafted an amendment to its ALU rules. Public hearings are set before the Planning Commission on October 29, 2026 and the Board of Supervisors on November 17, 2026, and the county has posted a staff report and FAQ. Staff recommends that the Board:

• drop the separate zoning administrative permit for by-right ALUs (building permits would still be needed in many cases);
• allow a detached ALU by right on lots of 36,000 sq ft or more, and with a special permit on smaller lots;
• set the maximum size for interior and detached ALUs at 1,200 sq ft or 40% of the main house, whichever is less;
• remove the owner-occupancy requirement;
• raise the occupancy limit to three people; and
• remove the extra parking space requirement and the renewal and recordation steps.

The draft also includes alternative options, such as a different minimum lot size for detached units, and keeping the current rules is also an option. These are proposals, not law, until the Board acts. Check the county's amendment page for the outcome. [3][7][8]

Does Virginia's 2027 ADU law apply here?

Virginia's ADU law, Va. Code 15.2-2292.4, takes effect July 1, 2027. It says "Nothing in this section shall apply to a locality that adopted an ADU ordinance prior to January 1, 2026." Fairfax County's staff report says the legislation therefore "does not apply to Fairfax County." The county's own ALU rules control, so ask the Zoning Administration Division if you have questions about your lot. [7][9]

Permit steps in Fairfax County

1. Confirm the county is your zoning authority

   If you are in the City of Fairfax or Falls Church, follow that city's rules. In the towns of Clifton, Herndon or Vienna, check with the town first; Clifton and Vienna projects need separate town approvals. [4][5]

2. Pick the right path

   An interior ALU that meets every standard in subsection 4102.7.B uses an administrative permit. A detached ALU, a larger interior unit, a front entrance or a missing parking space needs a special permit from the Board of Zoning Appeals. [1][2]

3. Gather your documents

   Prepare a plat showing the house and three parking spaces (four on a private street), a floor plan with each room labeled and measured, and dated interior and exterior photos. If an agent applies for you, add a signed owner permission letter certifying that the owner will live on the property. [2]

4. Apply in PLUS

   Submit the administrative permit through the PLUS application portal and pay the $285 fee. For PLUS help, call 703-324-2222. [2]

5. Get building permits

   File any building, electrical or plumbing permits in PLUS with Land Development Services. If you are on a well or septic system, get Health Department approval before the ALU permit is approved. [1][2][5]

6. Record the permit before anyone moves in

   Before occupancy, record a copy of the administrative permit in the Fairfax County land records, indexed under the owner's name. [1]

7. Renew it

   The permit runs for two years at first. Ask the Zoning Administrator to extend it, for up to five years at a time; the renewal fee is $75. [1][2]

Who to call

• Fairfax County Zoning Permits Section

  ALU administrative permit applications and questions (email dpdzoningpermits@fairfaxcounty.gov)

  703-222-1082

  [2]

• Fairfax County Zoning Administration Division

  ALU zoning ordinance and the proposed 2026 ALU amendment

  703-324-1314

  12055 Government Center Parkway, Suite 801, Fairfax, VA 22035

  [3][8]

• Fairfax County Land Development Services

  Building, electrical and plumbing permits and inspections

  703-222-0801

  12055 Government Center Pkwy, Suite 324, Fairfax, VA 22035

  [5]

Questions people ask