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Accessory Dwelling Opportunities Across Northern Virginia – A Comparison



Increasing the supply of affordable housing is a challenge for localities across Northern Virginia and throughout the region.  While local governments have focused a lot of effort on the traditional route of providing financial subsidies and incentives for the development of committed affordable units, other options include measures to increase housing density in order to put more housing on a limited supply of available land.

One option that is increasingly being recognized as an effective means of addressing the nationwide shortage of affordable rental housing is to allow accessory dwelling units in neighborhoods that are predominantly single-family. An accessory dwelling unit is defined as a smaller (subordinate to the primary home), independent residential dwelling unit located on the same lot as a single-family home. These may be known as “granny flats,” “mother-in-law apartments,” “carriage houses,” “guest houses,” “English basements,” or numerous other terms.

Over the past two years each of the major Northern Virginia jurisdictions have expanded the ability to build and use these small living spaces in predominantly residential neighborhoods. 

During the 2021 NVAR Convention and Trade Show, planning and zoning department staff from our local Northern Virginia jurisdictions joined us to discuss what is allowed and what Realtors® need to know about the use and ownership of accessory living spaces in Northern Virginia Homes.

A recording of that event may be viewed here: Accessorize It: All You Need to Know about Accessory Dwellings in Northern Virginia

As a follow-up to that conversation, NVAR has created a chart comparing the current zoning ordinances regulating accessory dwellings in Arlington, Alexandria and Fairfax.


Arlington County

Fairfax County

City of Alexandria

Ordinance/Code Section

Accessory Dwelling (AD).

Section §12.9.2 of the Zoning Ordinance.

Accessory Living Unit (ALU).

Subsection 4102.7.B of the Zoning Ordinance.

Accessory Dwelling (AD).

Section 7-203 of the Zoning Ordinance.

What is the Permit Approval Process?

Administrative permit application.

Administrative permit application for interior/attached ALUs.


Detached ALUs require a minimum of two acre lots and special permit approval from the Board of Zoning Appeals (BZA).

Administrative permit application.

What are the Fees for the Permit?

The administrative application and permit fee is $146 plus $311 Parking Survey (if required).

The administrative permit fee is $200, and the special permit fee is $435. The fee for a renewal is $70.

No Fee. (Currently there is no fee for submitting an administrative permit application. A fee of $50-$100 is expected to be proposed and approved in the coming months.)

What type of property may have an Accessory Dwelling?

Only single-family detached dwellings in R zoning Districts may have an AD.

Only single-family detached dwellings may have an accessory living unit.

An accessory dwelling shall be permitted as an accessory use to a single-family, two-family or townhouse dwelling.

What Restrictions are on the Size and Number of Accessory Units?

There cannot already be an accessory dwelling or a family/caregiver suite on the lot.


Detached accessory dwellings approved after May 18, 2019, may not exceed 25 feet nor 1½ stories in height.


No detached accessory building approved after May 18, 2019, with a footprint larger than 560 square feet in R-5 and R-6 districts, and 650 square feet in any other district, shall be used to contain an accessory dwelling.


The gross floor area of an accessory dwelling shall not exceed the following:

(1) The size of the basement for an attached accessory dwelling that is wholly within a basement.


(2) 750 sq. ft. nor 35 percent of the combined floor area of the main and accessory dwelling, for an attached or detached accessory dwelling where the GFA of the main dwelling is at least 1,000 sq. ft.


(3) 500 sq. ft. nor 45 percent of the combined gross floor area of the main and accessory dwelling, for an attached or detached accessory dwelling where

the GFA of the main dwelling is less than 1,000 sq. ft.

Only one accessory living unit is allowed per dwelling.


In an interior unit, the accessory unit size is limited to 800 square feet or 40% of the principal dwelling, whichever is less, except that the ALU may occupy the entire basement or cellar if the basement/cellar was existing as of July 1, 2021. In addition, the size also may be exceeded with special permit approval.


In a detached unit, the size is limited to 1,200 square feet. A larger size can not be approved by the BZA.

Only one accessory dwelling shall be permitted on any recorded lot.


The gross floor area of an accessory dwelling within a principal dwelling shall not exceed one-third of the principal dwelling's gross floor area or 350 square feet for lots 2,500 square feet or less and 500 square feet for lots larger than 2,500 square feet.


The maximum height of the accessory building shall not exceed the lesser of the following: the height of the principal dwelling; or 20 feet.

How many Persons may Occupy an Accessory Dwelling?

No more than three persons shall occupy the accessory dwelling.

A maximum of two people may live in an ALU, and the space is limited to no more than two bedrooms.

No more than three persons shall reside in the accessory dwelling.

Are Accessory Dwellings allowed as Attached or Detached Structures?

Accessory dwellings may be within or attached to one-family dwellings, or in detached accessory buildings.


Detached accessory buildings existing prior to May 18, 2019, may be altered to make interior alterations to create an accessory dwelling.

Interior/attached ALUs are allowed by administrative permit.


Only properties with at least two acres may apply for a detached accessory living unit. All detached units require a special permit application and a public hearing with the BZA.

Accessory dwellings may be within or attached to an existing home (single, two-family or townhouse), or within a detached accessory structure.


What are the Parking Space Requirements?

If no parking spaces exist prior to issuance of accessory dwelling permit, 1 space shall be created, provided, however, where a parking survey conducted by the County determines that the block on which the main dwelling is located is less than 65% parked, such new space is not required to be created.


Where either 1 or 2 spaces exist prior to issuance of the accessory dwelling permit, all such space(s)shall be maintained.


Where more than 2 spaces exist prior to issuance of the accessory dwelling permit, at least two spaces shall be


For a dwelling located on a public street, three total off-street parking spaces are required and on a private street, four total spaces are required for interior/attached ALUs. The parking spaces can be in a driveway, garage, carport, etc. but may not be located on the street. Front yard coverage for driveways is limited to 25 or 30 percent in the R-1 through R-4 Districts, depending on the district. For ALUs approved by special permit, the BZA will determine whether additional parking beyond the base requirement for a single-family detached dwelling (two spaces on a public street, three spaces on a private street) is required.

An accessory dwelling shall be exempt from providing off-street parking.

Is Owner Occupancy Required?

Yes. The applicant must be the owner of the property where the AD is proposed to be located and the property owner/applicant must use either the main dwelling or the AD as their primary residence.

Yes. The owner of the home must live on the premises either within the principal dwelling or the accessory living unit.

Yes, but the owner of the property shall only be required to maintain the property as their primary residence until after the permit is issued.


The accessory and principal dwelling shall remain under common ownership.

Are Rentals Allowed in Accessory Units?

Only one (either the principal or the accessory unit) may be rented out.


Accessory Homestays (short-term rental) shall be allowed in an attached or detached accessory dwelling only where such is approved as an accessory dwelling.

Rentals are permitted (subject to the occupancy provisions of subsection 4102.7.B(7) and (8)).


ALUs are prohibited from being used for short-term lodging purposes.

Both the principal and accessory unit may be rented after the permit has been issued.


An accessory dwelling may not be used as a short-term residential rental for a period exceeding 120 days per calendar year.

What additional Building Code and Design Requirements Exist for Accessory Units?


No accessory dwelling within a main dwelling with an entrance above the first floor shall have exterior stairs to that entrance on the side of the lot fronting a street.


No accessory dwelling within an accessory building with an entrance above the first floor shall have exterior stairs to that entrance on the side of the lot fronting a street unless the accessory building was built prior to May 18, 2019.


Any detached accessory building approved after May 18, 2019, containing an accessory dwelling shall comply with setbacks as follows:


On interior lots, the nearest wall may not be closer than five feet to a side or rear lot line;


On corner lots, the nearest wall may not be closer than five feet to any side lot line, and the nearest edge of the eave of the accessory building may not be closer than 10 feet to any rear lot line;


The nearest wall of the accessory building may not be closer than 25 feet from any street or officially designated street right-of-way line.

Emergency egress windows may be required for basement bedrooms. Any interior or exterior construction may require approval of a building permit. In addition, depending on the type of cooking device in the ALU, either fire rated floor and wall assemblies or a fire alarm system with third-party monitoring that complies with NFPA 72, National Fire Alarm and Signaling Code, may be required.


If the property is served by a well or septic system, Health Department approval will be required prior to an administrative permit or special permit being approved.


For interior/attached ALUs, any new external entrance must be located on the side or rear of the dwelling. Any proposed garage orcarportmust be located directly adjacent to any existing garage or carport, and the associated driveway and curb cut must be the same as that which serves the principal dwelling.

Unless a lesser setback is required by the zone in which the property is located, the accessory building shall provide at least a one-foot setback, including architectural features, from side and rear lot lines.


If the accessory building has walls with windows or doorways that face the nearest side or rear lot line, the setback shall be three feet unless a greater setback is required.


Portions of an accessory building taller than 13.5 feet but less than 16 feet shall provide a setback of at least 2.5 feet.


Portions of accessory buildings more than 16 feet in height shall provide a setback of at least five feet.

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