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Accessory Dwelling Units

This page provides a brief overview of accessory dwelling units for cities and counties in Washington State, including legal requirements and examples of city and county codes.

What is an Accessory Dwelling Unit?

An accessory dwelling unit (ADU) is a small, self-contained residential unit located on the same lot as an existing single-family home. An ADU has all the basic facilities needed for day-to-day living independent of the main home, such as a kitchen, sleeping area, and a bathroom. As the term "accessory" implies, ADUs are generally defined to be smaller in size and prominence than the main residence on the lot. Some definitions include specific size limits, and a location that is not readily visible from the street.

ADUs are sometimes called "mother-in-law apartments" or "granny flats" because they are often used to house extended family. Other codes use terms such as "accessory apartment," "accessory living unit," or "secondary unit," to have a similar meaning.

In theory, an ADU may be created as a separate unit within an existing home (such as in an attic or basement) an addition to the home (such as a separate apartment unit with separate entrance) or in a separate structure on the lot (such as a converted garage). Some communities, however, only allow ADUs that are within or attached to the main residence, and may define ADU to exclude an ADU housed in a separate structure. Whether attached or detached from the main residence, most codes require that the main residence and the ADU are owned by the same person, and may not be sold separately.

Accessory Dwelling Units in Washington State

RCW 43.63A.215 and RCW 36.70A.400, adopted as part of the 1993 Washington Housing Policy Act, require many Washington cities and counties to adopt ordinances encouraging the development of accessory apartments or accessory dwelling units in single-family zones. Specifically, this legislation applies to:

  • Cities with populations of over 20,000,
  • Counties with populations of over 125,000, and
  • Counties/cities that plan under the Growth Management Act (GMA).

Other communities may choose to allow ADUs if so desired.

Local codes must incorporate the model ordinance recommendations prepared by the State Department of Community, Trade and Economic Development (now Department of Commerce) (RCW 43.63A.215 and RCW 36.70A.400). However, state law allows local communities some flexibility to adapt these recommendations to local needs and preferences.

ADU ordinances have been widely adopted in Washington, since the 1993 Act. In part, this is because ADUs have helped local jurisdictions meet GMA goals to encourage affordable housing and provide a variety of housing densities and types, while preserving the character of single-family neighborhoods.

Examples of Local ADU Codes

Below are examples of accessory dwelling unit provisions adopted by cities and counties in Washington State, as well as two prominent out-of-state examples.

Medium and Large Cities

Small Cities



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Last Modified: April 10, 2017