Accessory Dwelling Unit Requirements in Arlington County

Arlington County allows one accessory dwelling unit (ADU) per lot in Residential (R) districts, provided the property owner occupies either the main house or the ADU as their primary residence. These units are limited to a maximum of three occupants and must comply with specific size, height, and setback standards based on whether the unit is internal to the main structure or detached.

General Standards for Accessory Dwellings

In Arlington County, accessory dwellings are permitted in Residential (R) districts subject to the issuance of a permit by the zoning administrator. These units may be located within or attached to a one-family dwelling, or within a detached accessory building on a lot containing a one-family dwelling [Section 12.9.2].

The following general conditions apply to all accessory dwellings in Arlington County:

Quantity: No more than one accessory dwelling is permitted on any single lot [Section 12.9.2.A].
Conflicts: An accessory dwelling is not permitted on any lot that already contains a family/caregiver suite [Section 12.9.2.A].
Occupancy: No more than three persons may occupy an accessory dwelling [Section 12.9.2.C].
Owner Occupancy: The owner of the property must occupy either the primary dwelling or the accessory dwelling as their primary residence (at least 185 days per year). If the owner does not reside on-site, the property may only be occupied by a single family [Section 12.9.2.C].
Covenants: Before a building permit is issued, the owner must record a covenant in the land records identifying the accessory dwelling use and the restrictions imposed by the Arlington County Zoning Ordinance [Section 12.9.2.C].

Size and Floor Area Requirements

The maximum gross floor area (GFA) for an accessory dwelling in Arlington County depends on the size of the main dwelling and the location of the unit.

Unit Type/Location	Maximum Floor Area (GFA)
Wholly within a basement	Limited only by the size of the basement [Section 12.9.2.A].
Units where Main Dwelling is ≥ 1,000 sq. ft.	Lesser of 750 sq. ft. or 35% of the combined GFA of the main and accessory dwelling [Section 12.9.2.A].
Units where Main Dwelling is < 1,000 sq. ft.	Lesser of 500 sq. ft. or 45% of the combined gross floor area of the main and accessory dwelling [Section 12.9.2.A].

Standards for Detached Accessory Dwellings

For detached accessory buildings in Arlington County approved after May 18, 2019, that contain an ADU, the following dimensional and placement standards apply:

Standard Category	Requirement
Maximum Height	25 feet or 1.5 stories [Section 12.9.2.A].
Footprint (R-5 and R-6)	Maximum 560 square feet [Section 12.9.2.A].
Footprint (All other districts)	Maximum 650 square feet [Section 12.9.2.A].
Street Setback	Minimum 25 feet from any street or officially designated right-of-way [Section 12.9.2.A].
Side/Rear Setback (Interior Lot)	Minimum 5 feet from a side or rear lot line [Section 12.9.2.A].
Side Setback (Corner Lot)	Minimum 5 feet from the side lot line [Section 12.9.2.A].
Rear Eave Setback (Corner Lot)	Minimum 10 feet from the rear lot line [Section 12.9.2.A].

Parking Requirements

Parking for accessory dwellings in Arlington County is regulated based on the existing parking available on the lot and the results of neighborhood parking surveys.

Existing Parking Status	Parking Requirement
No existing parking spaces	1 new space must be created, unless a County survey shows the block is less than 65% parked [Section 14.3.7].
1 or 2 existing spaces	All existing spaces must be maintained [Section 14.3.7].
More than 2 existing spaces	At least 2 existing spaces must be maintained [Section 14.3.7].

All new parking spaces must be constructed and maintained in accordance with Arlington County's general parking standards [Section 14.3.3]. Additionally, tandem parking spaces are permitted for accessory dwellings provided they are located on a driveway with an approved curb cut and meet minimum dimensions for full-size automobiles [Section 14.3.3.I].

Use Limitations and Prohibitions

Exterior Appearance: There shall be no evidence on the exterior of the premises that the property is used for anything other than a dwelling. No signs are permitted for the ADU [Section 12.9.11.C].
Exterior Stairs: For units within a main dwelling, no exterior stairs to an entrance above the first floor are permitted on the side of the lot fronting a street. For detached buildings, exterior stairs to a second floor are only permitted if the building was constructed before May 18, 2019 [Section 12.9.2.A].
EHO Developments: Accessory dwellings are permitted within or attached to semidetached or townhouse dwellings under Expanded Housing Option (EHO) rules, but they are expressly prohibited on lots containing duplex or multi-family dwellings subject to EHO provisions [Section 10.4.5.B].

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