Accessory Dwelling Unit Size Standards in Arlington County
In Arlington County, the maximum gross floor area allowed for an accessory dwelling unit is generally the lesser of 750 square feet or 35% of the combined gross floor area of the main and accessory dwellings. If the main dwelling in Arlington County is smaller than 1,000 square feet, the accessory unit is further restricted to the lesser of 500 square feet or 45% of the combined gross floor area. Accessory units created wholly within a basement are limited only by the total size of that basement [§12.9.2.A.2.e].
Maximum Size Standards for Accessory Dwellings
Arlington County regulates the size of accessory dwellings based on the type of unit and the square footage of the primary residence. These standards apply to units located within or attached to one-family dwellings, or within detached accessory buildings on lots containing one-family dwellings [§12.9.2.A.1].
The following table outlines the maximum gross floor area (GFA) allowed for accessory dwellings in Arlington County:
| Type of Accessory Dwelling | Size of Main Dwelling | Maximum Gross Floor Area (GFA) | Maximum Percentage of Combined GFA |
|---|---|---|---|
| Wholly within a basement (Attached) | Any | Size of the basement | N/A |
| Other Attached or Detached Units | 1,000 sq. ft. or more | 750 sq. ft. | 35% |
| Other Attached or Detached Units | Less than 1,000 sq. ft. | 500 sq. ft. | 45% |
Additional Dimensional Constraints
Beyond the total gross floor area, Arlington County imposes specific footprint and height limitations on detached accessory buildings intended for use as accessory dwellings if they were approved after May 18, 2019.
Footprint and Height Limits
For detached accessory units, the building footprint is capped based on the specific residential district within Arlington County. Furthermore, these detached structures are limited to a maximum height of 25 feet and 1.5 stories [§12.9.2.A.2.c].
| District in Arlington County | Maximum Detached Building Footprint |
|---|---|
| R-5 and R-6 Districts | 560 sq. ft. |
| All Other Districts | 650 sq. ft. |
Occupancy and Usage Requirements
Arlington County mandates that no more than one accessory dwelling is permitted per lot, and it cannot be located on a lot that already contains a family/caregiver suite [§12.9.2.A.1]. Additionally, the combined occupancy of the accessory dwelling is limited to a maximum of three persons [§12.9.2.C.2].
Sections Cited
- §12.9.2 Accessory dwellings
- §12.9.2.A.1 Standards
- §12.9.2.A.2.c Height and Stories
- §12.9.2.A.2.d Footprint
- §12.9.2.A.2.e Gross Floor Area
- §12.9.2.C.2 Conditions of Approval (Occupancy)