An accessory dwelling unit (ADU) is a smaller, typically independent residential dwelling unit located on the same lot as a stand-alone single family home. ADUs are typically detached from the primary residence on a lot, but may also be attached, such as in the case of converted garages, floors, or rooms of a house.
ADUs go by many names, including accessory apartment, accessory dwelling, "in-law suite," "granny flat," "Ohana units" (in Hawaii), and backyard cottages, among other regional variations.
Many cities and counties permit ADUs in one or more single-family zoning districts by right, subject to use-specific standards. Common provisions include an owner-occupancy requirement (for one of the two dwellings), dimensional and design standards to ensure neighborhood compatibility, and off-street parking requirements. Other relatively common provisions include minimum lot sizes and limits on the number of occupants or bedrooms. While some codes also include occupancy restrictions that stipulate that ADUs can only house family members or domestic employees, this type of restriction can severely limit the potential for ADUs to address a shortage of rental housing.
In some states, such as California and Vermont, localities must permit ADUs by right, under certain conditions. In some others, state laws pre-empt some aspects of local zoning for ADUs or actively encourage cities and counties to adopt permissive zoning regulations for ADUs.
Permitted and un-permitted ADUs are present throughout most residential areas of the United States.
In a 2016 McKinsey report, researchers estimated that California could add up to 790,000 housing units if homeowners are willing to adapt their properties to accommodate ADUs.
An interim report from the UC Berkeley Center for Community Innovation scoring various local ADU legislation in the state, published in February 2020, found that California's Department of Housing and Community Development had received 201 local municipal and county ordinances governing ADUs by September 6 2019. The report says that 540 municipalities and counties are expected to submit ordinances, representing 37.2 percent of the state.
Following liberalization of ADU permitting regulations in the state in 2016 and thereafter, the number of ADU permits issued grew significantly.
In Los Angeles alone, an average of 95 ADU permits were issued annually between 2003 and 2016. According to data published by the Los Angeles Department of City Planning, a larger and growing number of ADU permits were issued in 2017-2019:
- 3,195 permits in 2017 (1,550 to alter an existing structure, 1,126 to make an addition to an existing structure, and 519 permits for new construction)
- 4,625 permits in 2018 (2,030 to alter an existing structure, 1,669 to make an addition to an existing structure, and 896 permits for new construction)
- 4,693 permits in 2019 (1,922 to alter an existing structure, 1,766 to make an addition to an existing structure, and 1,005 permits for new construction)
