ADU/ALU Rules in Northern Virginia Counties — Simplified

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If you are considering an Accessory Dwelling Unit (ADU) in Northern Virginia, one of the first things you may discover is that there is no single set of Northern Virginia ADU rules.

Fairfax County calls them Accessory Living Units (ALUs). Arlington County uses Accessory Dwellings (ADs). Other jurisdictions use Accessory Dwelling Units (ADUs) or similar terminology.

More importantly, the rules can change dramatically simply by crossing a county or city line.

A basement apartment that may qualify as an accessory dwelling in one jurisdiction may be treated very differently in another. The same is true for an attached in-law suite, apartment over a garage, or detached backyard cottage.

This guide simplifies the rules for:

  • Fairfax County

  • Arlington County

  • Loudoun County

  • Prince William County

  • City of Alexandria

  • City of Falls Church

Rules verified August 2026. Zoning ordinances change, and individual properties can have additional restrictions. Always verify the zoning of a specific property before beginning design or construction.

First: What Is an ADU or ALU?

An ADU is essentially a secondary independent dwelling associated with a primary residence.

It normally contains its own:

  • Sleeping area

  • Bathroom

  • Living area

  • Cooking facilities

You may hear these spaces described as:

  • Accessory dwelling units

  • Accessory living units

  • In-law suites

  • Basement apartments

  • Backyard cottages

  • Garage apartments

  • Granny flats

These terms are often used interchangeably in everyday conversation, but they do not necessarily mean the same thing under zoning regulations.

That distinction matters.

Attached vs. Detached ADUs: The Most Important Difference

Before comparing jurisdictions, it helps to divide ADUs into two basic categories.

Interior / Attached ADU Detached ADU
What is it? A separate living unit inside or physically connected to the main house A separate residential structure on the same property
Common example Basement apartment Backyard cottage
Other examples In-law suite addition, converted portion of the home Detached garage apartment, small backyard house
Kitchen Typically yes Yes
Bathroom Yes Yes
Separate entrance Often Yes
Typical zoning difficulty Usually easier Usually more restrictive
Common issues Size, entrances, parking, occupancy Setbacks, lot coverage, height, utilities, parking and placement

For many Northern Virginia homeowners, the basement is the first place to investigate.

Why?

Because you are working within the existing principal dwelling rather than creating another residential building on the property. Several Northern Virginia jurisdictions also provide special treatment for ADUs located entirely within an existing basement.

If you are instead planning a separate backyard cottage or detached garage apartment, zoning becomes much more important.

For a broader explanation of how setbacks and buildable area work, see our Guide to Zoning and Setbacks in Northern Virginia.

Northern Virginia ADU Rules at a Glance

Rule Fairfax County Arlington County Loudoun County Prince William County Alexandria Falls Church
Local term ALU AD Accessory Dwelling ADU ADU No comparable general residential ADU program identified
Interior / attached unit? ✓ Yes ✓ Yes ✓ Yes ⚠ Not as an independent second dwelling under the standard second-kitchen rules ✓ Yes ✓ Yes
Basement unit? ✓ Yes ✓ Yes ✓ Potentially ⚠ Second kitchen may be possible, but not as a separate/private independent living area ✓ Yes ✓ Yes
Detached ADU? ✓ Only under restrictive current rules ✓ Yes ✓ Yes, subject to zoning standards Generally no conventional detached residential ADU on a typical residential lot ✓ Yes ✓ Yes
Special basement size treatment? ✓ Yes ✓ Yes Depends on district/structure N/A Property-specific ✓ Yes
Typical maximum size Interior: 800 SF or 40%, whichever is less, with qualifying basement exception. Detached: 1,200 SF Usually up to 750 SF or 35% for qualifying homes; basement exception applies Varies by zoning district; suburban districts generally capped at 1,200 SF and proportional limits N/A for conventional ADU Depends on applicable zoning standards Basement: up to basement area. Other ADs: generally up to 1,000 SF or 50% for larger homes; 700 SF for smaller homes
Owner occupancy? ✓ Yes ✓ Yes, subject to ordinance provisions Check applicable zoning/property conditions N/A Check current applicable requirements ✓ Yes
Detached minimum lot size 2 acres under current rules No Fairfax-style 2-acre minimum Depends on zoning district N/A Depends on underlying zoning No Fairfax-style 2-acre minimum
Extra zoning review may be required? ✓ Yes ✓ Yes ✓ Depends on property ✓ Definitely investigate before designing ✓ Depends on project ✓ Depends on project

Important: This table is designed as a homeowner's quick reference, not as a substitute for a zoning determination. ADU feasibility can also depend on zoning district, setbacks, lot coverage, utilities, septic capacity, easements, building code requirements and existing improvements.

Fairfax County ALU Rules — Simplified

Fairfax County officially calls an accessory dwelling an Accessory Living Unit, or ALU.

The easiest way to understand Fairfax County's current rules is to ask:

Is the ALU inside the house?

If the ALU is located within the principal single-family detached dwelling and meets the applicable standards, it may generally be approved through an Administrative Permit.

Under the current rules, an interior ALU is generally limited to:

800 square feet or 40% of the gross floor area of the principal dwelling, whichever is less.

However, there is an important exception.

Fairfax County's Basement Exception

The entirety of a qualifying basement or cellar may potentially be used for the ALU, subject to Fairfax County's rules regarding the basement's applicable existing size.

That makes basement conversions particularly important when evaluating ALU possibilities in Fairfax County.

Other important current requirements include:

  • Only a single-family detached dwelling may have an ALU.

  • The ALU may contain no more than two bedrooms.

  • No more than two people may occupy the ALU.

  • Either the principal dwelling or ALU must be owner-occupied.

  • An additional off-street parking space is generally required for an administratively approved interior ALU.

  • Applicable entrance and access requirements must be satisfied.

For the detailed rules, see our Complete Guide to Accessory Living Units in Fairfax County.

What About a Detached ALU in Fairfax County?

This is where Fairfax becomes considerably more restrictive.

Under the rules in effect as of August 2026, a detached ALU requires:

  • A lot of at least 2 acres

  • Approval of a Special Permit

  • A maximum detached ALU gross floor area of 1,200 square feet

  • Compliance with applicable zoning and development requirements

What This Means for a Typical Fairfax County Homeowner

If your property is on a typical suburban lot under two acres, do not start by designing a detached backyard ADUunder the current rules.

Investigate an interior ALU first—particularly a basement—or determine whether an attached living-space solution can satisfy your family's needs.

Fairfax County is currently evaluating changes to its ALU regulations, so homeowners considering future projects should verify the rules again before proceeding.

Arlington County ADU Rules — Simplified

Arlington calls these units Accessory Dwellings, or ADs.

Arlington allows an AD:

  • Within the main dwelling

  • Attached to the main dwelling

  • Within a qualifying detached accessory building

  • In a newly constructed detached accessory building that complies with the applicable standards

That last item is an important difference from Fairfax County.

Arlington Basement ADUs

Arlington provides particularly favorable treatment for an AD located wholly within a basement.

There is no specific AD size limit for a unit located entirely within the basement; the basement's perimeter establishes the practical boundary.

For many other ADs where the existing main dwelling is at least 1,000 square feet, the AD is generally limited to:

35% of the combined floor area of the main dwelling and AD, up to 750 square feet.

Different limits apply to smaller principal dwellings.

Detached ADUs in Arlington

New detached ADs are possible, but this does not mean that any backyard can accommodate one.

Depending on the zoning district and property, requirements can regulate:

  • Building footprint

  • Height

  • Side and rear setbacks

  • Distance from the principal dwelling

  • Distance from the street

  • Lot coverage

  • Parking

For example, Arlington limits the footprint of newer accessory buildings containing an AD to 560 square feet in R-5 and R-6 districts and 650 square feet in several larger-lot residential districts.

What This Means for an Arlington Homeowner

Arlington can provide more options for a detached ADU than Fairfax County, but the property's buildable envelope is critical.

On Arlington's smaller residential lots, setbacks, coverage and the existing house can determine feasibility long before the interior floor plan does.

Loudoun County Accessory Dwelling Rules — Simplified

Loudoun County takes another approach.

Accessory dwellings can be located within the principal structure or within an accessory building, but allowable size varies significantly by zoning district.

For example, under Loudoun's zoning standards:

Suburban Zoning Districts

The accessory dwelling's gross floor area generally cannot exceed the lesser of:

  • 50% of the principal structure's gross floor area, or

  • 1,200 square feet

The footprint also cannot exceed 50% of the principal structure's footprint.

Urban Zoning Districts

The maximum generally becomes the lesser of:

  • 70% of the principal structure's gross floor area, or

  • 1,200 square feet

Rural, Transition and Certain Other Districts

The rules can be substantially more generous, with some accessory dwellings potentially reaching 2,500 square feet, subject to proportional limits and the applicable zoning standards.

Properties using individual sewage disposal systems also require Health Department approval, while properties using certain community wastewater systems may require approval from Loudoun Water.

What This Means for a Loudoun County Homeowner

Do not ask only:

“Does Loudoun allow ADUs?”

Ask:

“What is my Loudoun zoning district?”

The answer can materially change how large the accessory dwelling can be and what type of structure may accommodate it.

Prince William County — A Very Important Difference

Prince William County deserves its own explanation because homeowners should not assume the Fairfax, Arlington or Loudoun model applies here.

Prince William County allows what it calls a Secondary Food Preparation Area under specific circumstances in a single-family detached dwelling or attached addition.

But the County explicitly states that the area containing the second kitchen cannot be used or represented as a private, separate or independent living area under those provisions.

In other words:

A second kitchen does not automatically create a legal ADU.

The secondary food-preparation area must remain for the convenience and use of the residents of the dwelling rather than creating an independently rentable second residence.

Prince William does have other specialized residential provisions—for example, tenant dwellings associated with qualifying bona fide agricultural uses—but those should not be confused with a general suburban homeowner ADU program.

What This Means for a Prince William Homeowner

If your goal is an independent apartment, rentable basement unit, or detached backyard dwelling, verify zoning before spending money on ADU plans.

If your goal is simply to create an in-law area for family members within the house, a design without a legally separate independent dwelling may be possible—but the layout and use need to comply with Prince William County's requirements.

City of Alexandria ADU Rules — Simplified

The City of Alexandria allows Accessory Dwelling Units and recognizes several forms, including:

  • Basement apartments

  • Converted accessory spaces

  • Attached units

  • Detached structures

Alexandria permits ADUs as an accessory use with qualifying residential dwellings, and the City's ADU program has been in place since 2021.

This is important because Alexandria should not be treated as if it follows Fairfax County's rules simply because both are in Northern Virginia.

Alexandria has its own zoning ordinance.

What This Means for an Alexandria Homeowner

Both attached and detached ADUs can potentially be viable, but the underlying zoning and property characteristics still control the actual project.

Before designing a detached unit, determine:

  • Property zoning

  • Existing building coverage

  • Required yards and setbacks

  • Location of existing structures

  • Parking requirements

  • Historic district requirements, if applicable

  • Building-code implications

City of Falls Church ADU Rules — Simplified

Falls Church recently expanded its accessory dwelling rules, making this one of the most interesting jurisdictions in Northern Virginia for ADUs.

Accessory dwellings can take the form of:

  • A detached accessory building

  • A unit within the principal dwelling

  • An attached addition

  • A basement apartment

For qualifying properties, the City allows ADs subject to zoning and building requirements.

Falls Church Size Limits

If the AD is entirely within a basement, it cannot exceed the gross floor area of that basement.

For other ADs:

Principal dwelling of 1,400 square feet or more

The AD may generally be no larger than the lesser of:

  • 1,000 square feet, or

  • 50% of the principal dwelling's gross floor area

Principal dwelling under 1,400 square feet

The AD is generally limited to:

700 square feet.

Detached ADUs in Falls Church

Detached ADs are possible, but placement matters.

Among the current standards:

  • A detached AD cannot be located in the front yard or closer to the street than the principal dwelling.

  • A detached AD up to 15 feet/one story generally requires an 8-foot side and rear setback.

  • Taller detached ADs generally require 10-foot side and rear setbacks.

  • The detached AD must generally remain at least 5 feet from the primary dwelling.

  • Applicable lot-coverage requirements still apply.

The property owner must use either the principal dwelling or AD as the primary residence.

What This Means for a Falls Church Homeowner

Unlike Fairfax County, you do not need a two-acre property simply to pursue a detached accessory dwelling.

That makes detached ADs potentially feasible on significantly smaller properties—but the available yard, setbacks and lot coverage still determine whether the project physically fits.

ADU vs. In-Law Suite: They Are Not Necessarily the Same Thing

This distinction causes a lot of confusion.

Suppose you build an addition containing:

  • Bedroom

  • Full bathroom

  • Sitting room

  • Closets

  • Private exterior entrance

Is it automatically an ADU?

Not necessarily.

Now add a complete independent kitchen and configure the space so someone can live independently from the rest of the house.

The zoning analysis may change considerably.

An in-law suite can simply be additional living space within the primary residence. An ADU/ALU is a legally recognized secondary dwelling unit subject to the jurisdiction's requirements.

If your primary goal is creating private space for parents or extended family rather than establishing a completely independent dwelling, see our In-Law Suite Addition Guide.

Which Type of ADU Should You Investigate First?

You Have an Existing Basement

Start there.

Basements receive particularly useful treatment under the rules in Fairfax County and Arlington County, and they eliminate many of the site-planning problems associated with adding another structure to the property.

That does not mean a basement automatically qualifies.

You still need to evaluate items such as:

  • Ceiling height

  • Emergency escape and rescue openings

  • Stairs

  • Natural light

  • Plumbing

  • HVAC

  • Electrical service

  • Fire separation

  • Parking

  • Zoning

  • Building permits

But from a zoning and construction standpoint, an existing basement can often be the logical first option to investigate.

You Want Private Space for Aging Parents

First determine whether you actually need a legally independent dwelling unit.

An attached in-law suite may accomplish your objective without creating the same zoning issues as a fully independent ADU.

For example, an addition could provide a bedroom, accessible bathroom, sitting area and private entrance while maintaining functional integration with the main house.

This is why the intended use should be established before the plans are drawn.

You Want a Backyard Cottage

Check zoning before design.

This is one of the situations where jurisdiction makes an enormous difference.

A detached ADU may potentially be feasible in Arlington, Alexandria, Loudoun or Falls Church under applicable rules, while current Fairfax County rules require at least two acres and Special Permit approval.

The same idea can therefore be relatively straightforward on one Northern Virginia property and impossible under the standard rules a few miles away.

You Want an Apartment Above a Detached Garage

Do not assume that because a detached garage is permitted, a dwelling above it is also permitted.

Adding sleeping, cooking and sanitation facilities can change the structure's zoning and building-code classification.

If a garage is part of your project, see our Garage Addition Guide, but have the proposed residential use reviewed separately.

Before Designing an ADU, Check These 8 Things

1. Jurisdiction

Determine which county or independent city actually controls the property.

A Falls Church mailing address, for example, does not necessarily mean the property is inside the City of Falls Church. Many Falls Church addresses are actually governed by Fairfax County.

That distinction can completely change the ADU rules.

2. Zoning District

Find the property's exact zoning classification.

3. Interior, Attached or Detached?

Decide which type of ADU you are considering.

4. Existing House and Basement

Document the home's existing square footage and basement configuration.

5. Lot Size

This is especially important in Fairfax County because of the current two-acre requirement for detached ALUs.

6. Setbacks and Lot Coverage

Determine the actual buildable envelope before designing a detached ADU or addition.

Our Northern Virginia Zoning and Setbacks Guide explains this process in more detail.

7. Water and Sewer

Properties on private well or septic systems can face additional limitations based on bedroom count and system capacity.

8. Building-Code Feasibility

Zoning approval answers whether and where you may create the dwelling.

The building code determines how it must be constructed safely.

Those are two separate questions.

Fairfax County ADU Guides by Community

If your property is in Fairfax County, we have more detailed guides for several Northern Virginia communities:

Remember that a mailing address does not always identify the governing jurisdiction, particularly around Vienna and Falls Church. Verify the actual jurisdiction before relying on any local guide.

The Simplest Way to Think About Northern Virginia ADU Rules

If you remember nothing else from this guide, remember these four points:

1. Start with the jurisdiction.
There is no single Northern Virginia ADU rule.

2. A basement is often the first option worth investigating.
Several jurisdictions provide more flexibility for ADUs contained entirely within the principal dwelling or basement.

3. Detached ADUs require much more zoning analysis.
Lot size, setbacks, height, lot coverage and placement can determine whether the project works.

4. An in-law suite is not automatically an ADU.
The presence of independent cooking, sleeping and sanitation facilities—and how the space is designed and used—can change its zoning treatment.

Planning an ADU, ALU or In-Law Suite in Northern Virginia?

The best place to begin an ADU project is not with a floor plan.

Begin by determining what the property allows.

Once the jurisdiction, zoning district, lot limitations and type of living space are understood, you can make an informed decision between:

  • Converting a basement

  • Building an attached in-law suite

  • Expanding the existing house

  • Building above or alongside a garage

  • Pursuing a detached accessory dwelling

Home Perfection Contracting works with homeowners throughout Northern Virginia on home additions, in-law suites, basement conversions and accessory living spaces.

For projects involving new construction or an addition, our Step-by-Step Home Addition Process for Fairfax and Arlington explains what happens from initial zoning feasibility through permits, construction and final inspection.

If you are considering an ADU or ALU, contact Home Perfection Contracting to discuss your property and the type of living space you are considering.

Official ADU/ALU Resources

Because zoning regulations change, homeowners should always confirm current requirements directly with the applicable jurisdiction:

Last zoning review: August 2026.

This guide is intended to simplify publicly available zoning information for homeowners and is not a zoning determination or legal opinion. Requirements should be confirmed with the applicable jurisdiction for a specific property before construction or design.