Last updated: October 2026. Fairfax County is reviewing a zoning amendment that would change these rules. Public hearings are scheduled for October 29, 2026 (Planning Commission) and November 17, 2026 (Board of Supervisors). We will update this guide after each hearing.
An in-law suite can let a parent move closer, give an adult child a place to start out, or create flexible space for a caregiver. In Fairfax County, the zoning term for this kind of space is an accessory living unit (ALU). This guide explains the rules that apply today, what the proposed amendment would change, and how to plan a basement suite or addition so it is ready for permits.
This is a planning guide, not legal advice. Zoning rules change, and every lot is different. Always confirm requirements with Fairfax County before you design or build.
What Is an Accessory Living Unit?
Fairfax County defines an accessory living unit as a secondary dwelling unit established in conjunction with, and clearly subordinate to, a single-family detached house. It includes space for living, sleeping, eating and sanitation. The county notes that an ALU is sometimes called an in-law suite or accessory dwelling. It can be interior (inside or attached to the house, such as a finished basement or an addition) or detached (a separate structure on the lot).
The current standards are in subsection 4102.7.B of the Fairfax County Zoning Ordinance.
Does this apply to my address? These are Fairfax County rules. The City of Fairfax and the towns inside the county, such as Herndon, Vienna and Clifton, have their own zoning ordinances. If your home is in one of those jurisdictions, check with that town or city first.
Current Fairfax County ALU Rules (October 2026)
Today, an interior ALU that meets all of the standards can be approved with an administrative permit. If it does not meet them, or if the unit is detached, it needs a special permit approved by the Board of Zoning Appeals (BZA).
Key standards for an interior ALU (administrative permit)
- House type: only with a single-family detached home, and only one ALU per lot. ALUs are not currently allowed in townhouses.
- Interior connection: the unit must be wholly within the house, with direct access to the main home through a finished, temperature-controlled, fully enclosed interior space.
- Entrances: any new external entrance must be on the side or rear of the house.
- Size: no more than 800 square feet or 40% of the main dwelling’s gross floor area, whichever is less. Alternatively, the entire basement or cellar may be used, up to its size on a cutoff date set in the ordinance. The county’s permit page lists July 1, 2021; confirm the current cutoff date with the county before you plan around it. Basement space with at least 6 feet 6 inches of headroom counts toward floor area; a garage does not.
- Bedrooms and occupancy: up to two bedrooms and no more than two people living in the ALU.
- Owner occupancy: the owner must live in either the main house or the ALU.
- Parking: the parking required for the house plus one additional space. The application plat must show three spaces on a public street or four on a private street, and those spaces must stay available. For example, a garage counted as parking cannot be used for storage.
- Safety: a working multi-purpose fire extinguisher, and smoke and carbon monoxide detectors (when required) interconnected with the main house.
- Recording and renewal: the permit must be recorded in the county land records before anyone moves in. The first approval lasts two years and can be extended for periods of up to five years.
Detached ALUs today
A freestanding ALU currently needs a special permit from the BZA and a lot of at least two acres. A detached unit may not exceed 1,200 square feet of gross floor area, and it must be clearly subordinate to the main home.
How the permit process works today
- Applications are submitted online through the county’s PLUS portal.
- The county says processing may take about 30 calendar days. If a building permit is needed, the zoning approval waits until the building permit passes final inspection.
- The application fee is $285, and a renewal is $75. A 2.35% service charge applies to card payments.
- You will typically need a plat showing the house and parking, a labeled floor plan with square footage and entrances, interior and exterior photos for an existing home, and an owner permission letter if an agent applies on your behalf.
- If the home uses a well or septic system, the Health Department must approve the added bedrooms.
What the Proposed 2026 Amendment Would Change
On September 29, 2026, the Board of Supervisors authorized public hearings on an ALU zoning amendment. According to the county’s staff materials and FAQ (October 2, 2026), the proposal would:
- Remove the separate administrative permit for interior ALUs, and for detached ALUs on lots of 36,000 square feet or more. Zoning review would happen as part of the related building permit.
- Remove the renewal requirement, the land-records recordation requirement, the owner-occupancy requirement and the extra parking space.
- Allow detached ALUs by right on lots of 36,000 square feet or more, and with special permit approval on smaller lots. The Planning Commission and Board may choose a different minimum lot size, from no minimum up to the current two acres. Another option under consideration would limit the special permit path to lots of at least 15,000 square feet.
- Change the size limit to 1,200 square feet or 40% of the main dwelling, whichever is less. An ALU could also occupy the entire basement or cellar. A larger size could be approved by the BZA with a special permit.
- Raise occupancy from two to three people, while keeping a limit of one or two bedrooms.
- Define a “semi-detached” unit: a unit connected by a hallway or enclosed breezeway of ten feet or less would count as interior. Longer connections would be treated as detached.
- Townhouses: staff did not recommend a change, but an option to allow interior ALUs in single-family attached homes with a special permit is included for consideration.
Some things would not change. Building permits and inspections would still be required. A property with an ALU still could not also be used for short-term lodging, such as Airbnb. Setbacks, coverage limits, stormwater and tree conservation rules would still apply.
Important: These are proposals. The final rules depend on the votes at the hearings, and the effective date has not been announced. Do not design around the proposed rules until the Board adopts them.
Key dates
- Planning Commission public hearing: Thursday, October 29, 2026, 7:30 p.m.
- Board of Supervisors public hearing: Tuesday, November 17, 2026, 4:30 p.m.
- Both hearings take place in the Government Center auditorium, 12000 Government Center Parkway, Fairfax.
Planning an In-Law Suite: Basement, Addition or Detached?
Basement suites
A finished basement is often the most practical place for an interior ALU, because the space and the interior connection already exist. The design work usually focuses on egress windows for bedrooms, ceiling height, moisture control, a bathroom, and a kitchen or kitchenette. Our guides to basement bedroom conversions and egress and adding a basement kitchen cover these details. You can also see our basement remodeling services.
Home additions
An addition can create a ground-floor suite with step-free access, which matters for aging parents. It also brings in setbacks, lot coverage, structural design and more permits. See our home addition services and home additions in Fairfax, VA.
Detached units
Under the current rules, a detached ALU needs a two-acre lot and a special permit, so it is not an option for most Fairfax County lots today. If the amendment is adopted as proposed, more lots would qualify. Grading plans, tree protection and stormwater rules may apply.
Design for accessibility
If the suite is for an older family member, plan for wider doorways, a curbless shower, grab-bar blocking and good lighting from the start. Our aging-in-place bathroom guide explains the options.
Permits You May Need Beyond Zoning
Zoning approval is only one step. Depending on the project, Fairfax County may require building, electrical, plumbing and mechanical permits. The county specifically mentions permits for a stove or other cooking appliance, emergency egress for a bedroom, and electrical work that was done without a permit in the past. Projects that disturb more than 2,500 square feet of land need a grading plan. For kitchen and bathroom work in the suite, see our Fairfax County kitchen permit guide.
A Simple Planning Checklist
- Confirm your jurisdiction: Fairfax County, City of Fairfax, or a town.
- Confirm the house type (single-family detached) and your lot size.
- Measure the gross floor area of the main house and the planned suite.
- Check headroom, egress windows and the interior connection.
- Count the off-street parking spaces you can show on a plat.
- Check well and septic if you are not on public water and sewer.
- Decide whether to apply under the current rules or wait for the amendment.
- Plan building permits together with zoning, not after.
Frequently Asked Questions
Is an in-law suite legal in Fairfax County?
Yes, as an accessory living unit, if it meets the zoning standards. Today an interior ALU in a single-family detached home can be approved with an administrative permit. Anything outside the standards needs a special permit from the Board of Zoning Appeals.
How big can an in-law suite be in Fairfax County?
Under the current rules, an interior ALU can be up to 800 square feet or 40% of the main house’s gross floor area, whichever is less. It may also use the whole basement or cellar, up to its size on the ordinance’s cutoff date. The proposed amendment would raise the limit to 1,200 square feet or 40%, whichever is less.
Do I have to live on the property?
Today, yes. The owner must live in either the main house or the ALU. The proposed amendment would remove this requirement.
Can I build a detached in-law cottage?
Currently only with a special permit, on a lot of at least two acres, up to 1,200 square feet. The proposal would allow detached ALUs by right on lots of 36,000 square feet or more, and with a special permit on smaller lots. The final lot size is still under discussion.
Can I rent out the ALU on Airbnb?
No. According to the county, short-term lodging is not allowed on a property with an ALU.
Can I add an ALU to a townhouse?
Not under the current rules. The amendment includes an option for interior ALUs in townhouses with a special permit, but staff did not recommend it. Watch the hearing results.
How long does approval take, and what does it cost?
The county estimates about 30 calendar days for the administrative permit, with a $285 application fee. Building permits and construction time come on top of that.
Plan Your In-Law Suite with Elegant Kitchen and Bath
Elegant Kitchen and Bath remodels basements, builds home additions and renovates kitchens and bathrooms across Fairfax County and Northern Virginia. We can review your space, sketch layout options and help you plan the permit path. Visit our Herndon showroom at 2445 Centreville Rd STE 46, Herndon, VA 20171, call (703) 763-4277, or request a free estimate. Also see our Fairfax remodeling page.