Accessory Dwellings


Accessory dwellings are additional, often small, housing units located within or adjacent to single-family homes. They are frequently known by more descriptive names such as basement apartments, granny flats, backyard cottages, in-law suites, and carriage houses. A property owner with an accessory dwelling can live in either the main dwelling or the accessory dwelling while renting out the other unit.

Arlington County adopted an ordinance in 2008 that allows accessory dwellings generally no greater than 750 square feet within a single-family home. Detached accessory dwellings – such as backyard cottages or detached carriage houses – are not currently allowed in Arlington.

Accessory dwellings are one way of adding to the supply of housing in single-family neighborhoods without altering the character of the area. Because of their size and characteristics, these units are often rented at rates that can be affordable to low- or moderate-income individuals.


2017 ADU Ordinance Updates

Since the Accessory Dwellings ordinance was enacted in 2009, there have been only 20 AD permits approved. Early in 2017 Arlington County identified possible adjustments to the existing ordinance as a goal that would advance the objectives of the Affordable Housing Master Plan. Under the Master Plan updates to the current AD regulations were shown to be a tool that could help increase the supply of affordable housing, especially to support seniors as they age in place.  A Working Group made up of members of the Housing Commission, the Commission on Aging, the Civic Federation, and the Alliance for Housing Solutions was created to assist the county in encouraging greater use of ADs.

In May, Arlington County held a community forum on the upcoming Accessory Dwellings update.  During the May forum, community members discussed some preliminary ideas that have come out of the Working Group. Issues raised include size and occupancy, design guidelines, compliance and parking, and detached ADs and townhomes. Read more about the preliminary ideas here.

Accessory Dwellings are a vital part of the Missing Middle Housing that would provide much needed access to affordable housing, especially for older adults as a way to age in place.

For more information about the Accessory Dwellings Regulation update visit the county page.


 Learn More

  • The website provides numerous examples of real accessory dwellings with photos, floor plans, and owner stories.
  • Arlington’s accessory dwelling permit process is managed by the zoning office of the Department of Community Planning, Housing, and Development.