Arlington County Accessory Dwelling Unit and Second Unit Guide
In Arlington County, accessory dwelling units (ADUs) are permitted as-of-right in all Residential (R) districts, provided the lot contains a one-family detached dwelling. Additionally, the Arlington County Zoning Ordinance allows for the creation of "second units" via duplexes in the R2-7 and R-5 districts, as well as through the Expanded Housing Option (EHO) which permits duplexes and small multi-family structures in the R-20, R-10, R-8, R-6, and R-5 districts.
Accessory Dwelling Units (ADUs) in Residential Districts
Arlington County defines an accessory dwelling as a complete independent dwelling unit with its own kitchen and bath, located on the same lot as a main one-family dwelling and under the same ownership [§18.2]. Arlington County allows one accessory dwelling per lot in all Residential (R) districts, subject to specific standards and the issuance of a permit by the Zoning Administrator [§12.9.2.A.1].
Permitted Districts for Accessory Dwellings
| District Name | District Classification | Permission Type |
|---|---|---|
| R-20 | One-family Dwelling District | Permitted (P) |
| R-10 | One-family Dwelling District | Permitted (P) |
| R-10T | One-family and Townhouse Dwelling District | Permitted (P) |
| R-8 | One-family Dwelling District | Permitted (P) |
| R-6 | One-family Dwelling District | Permitted (P) |
| R-5 | One-family and Restricted Two-family Dwelling District | Permitted (P) |
| R15-30T | Townhouse Dwelling District | Permitted (P) |
| R2-7 | Two-family and Townhouse Dwelling District | Permitted (P) |
| [Section 5.1.4] |
Size and Development Standards for ADUs
In Arlington County, the size of an accessory dwelling is limited based on the size of the main dwelling. Detached ADUs are subject to further footprint and height restrictions to maintain the residential character of the neighborhood.
| Standard Category | Requirement/Limit |
|---|---|
| Maximum Units | One per lot; cannot be on a lot with a family/caregiver suite [§12.9.2.A.1] |
| Max Floor Area (Attached - Basement) | Limited only by the size of the basement [§12.9.2.A.2] |
| Max Floor Area (General) | The larger of 750 sq. ft. or 35% of combined GFA (if main house is ≥ 1,000 sq. ft.) [§12.9.2.A.2] |
| Max Floor Area (Small Main House) | The larger of 500 sq. ft. or 45% of combined GFA (if main house is < 1,000 sq. ft.) [§12.9.2.A.2] |
| Max Footprint (Detached) | 560 sq. ft. (R-5, R-6); 650 sq. ft. (all other R districts) [§12.9.2.A.2] |
| Max Height (Detached) | 25 feet and 1.5 stories [§12.9.2.A.2] |
| Minimum Setbacks (Detached) | 5 ft. side/rear; 25 ft. from any street [§12.9.2.A.2] |
| Occupancy Limit | Maximum of three persons [§12.9.2.C.2] |
Second Units and Two-Family Dwellings
Arlington County provides two primary paths for "second units" that are not considered accessory (i.e., duplexes or two-family dwellings).
Standard Two-Family Districts
In Arlington County, the R2-7 District and R-5 District are the primary zones where two-family dwellings are recognized as a principal use.
- R2-7 District: Duplexes are permitted as-of-right [§5.1.2].
- R-5 District: Duplexes are permitted by special exception if the site abuts RA, C, or M districts or is located on a principal or minor arterial street [§5.7.4.A].
Expanded Housing Option (EHO)
Arlington County established the Expanded Housing Option (EHO) to increase the range of housing choices. This allows for duplexes (two units) and small multi-family buildings (up to six units) in several districts originally intended for one-family detached dwellings [§10.4.1].
| District Name | Allowed EHO Uses | Minimum Site Area (2-4 units) |
|---|---|---|
| R-20 | Duplex, Semidetached, Townhouse, Multi-family | 20,000 sq. ft. |
| R-10 | Duplex, Semidetached, Townhouse, Multi-family | 10,000 sq. ft. |
| R-8 | Duplex, Semidetached, Townhouse, Multi-family | 8,000 sq. ft. |
| R-6 | Duplex, Semidetached, Townhouse, Multi-family | 6,000 sq. ft. |
| R-5 | Duplex, Semidetached, Townhouse, Multi-family | 5,000 sq. ft. |
| [Section 10.4.2, Section 10.4.4] |
EHO Restrictions: Properties located within a "planning district" identified on the General Land Use Plan Map are not eligible for EHO development [§10.4.2.A]. Furthermore, accessory dwellings are explicitly prohibited on lots containing EHO duplex or multi-family dwellings [§10.4.5.B.2].
Sections Cited
- §5.1.2 Residential (R) Districts Principal Use Table
- §5.1.4 Residential (R) Districts Accessory Use Table
- §5.7.4 R-5 District Use Standards
- §10.4 Expanded Housing Option Development
- §10.4.1 EHO Purpose
- §10.4.2 EHO Applicability
- §10.4.4 EHO Density and Dimensional Standards
- §10.4.5 EHO Use Standards
- §12.9.2 Accessory Dwellings
- §18.2 General Terms Defined