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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.

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). See the examples shown below.

Some communities, however, only allow ADUs that are within or attached to the main residence, and exclude ADUs housed in a separate structure. Whether attached or detached from the main residence, most codes require that the main residence and the ADU must be owned by the same person and may not be sold separately.

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.

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 ADUs in single-family zones. Specifically, this legislation applies to:

  • Cities with a population over 20,000
  • Counties with a population over 125,000
  • Counties 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 Washington State Department of Community, Trade and Economic Development (now Department of Commerce), per 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 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

The table below provides examples of accessory dwelling unit codes adopted by cities and counties in Washington State, including a basic comparison of their provisions. (Click on the image to see a larger version.)

Most of these examples allow for both attached ADUs (sometimes called AADUs) and detached ADUs (sometimes called DADUs). With the exception of Vancouver, each requires that one of the units be occupied by the owner of the property. Many of the provisions limit the size of the accessory unit, ranging from 600 to 1,000 square feet. Some also limit the size of the accessory unit relative to the primary unit, ranging from 40 to 75 percent of the area of the primary unit.

Almost all of the code provisions rely on the development regulations of the underlying zones. Several provide maximum heights, onsite-parking, setbacks and other requirements specific to ADUs. Additionally, many of code provisions require the ADU to be similar in design to the primary unit (which may or may not be desirable, depending upon the design of the primary residence).

Washington Examples

Click on the table below to view a summary analysis of selected ADU provisions from cities in Washington State, including links to the actual code provisions.


Out-of-State Examples

Emerging Trends

A few Washington State cities, such as Seattle, are re-examining some of the standards listed above to determine if they are acting as barriers to the construction of new ADUs (see Seattle study process). One of the alternatives being examined by Seattle’s study is streamlining permitting by providing pre-approved ADU plans.

As may be seen in the out-of-state examples above, some cities such as Portland (OR), Santa Cruz (CA), and Vancouver (BC) have actually changed some of their requirements, such as eliminating owner occupancy, in order to encourage the production of more ADUs. Vancouver has also had success getting banks to provide lending products tailored to ADU projects that take into account the borrowers’ future rental income from the new unit. Portland also provides an ADU Financing Guide that identifies local financial institutions with programs that can be used to fund ADU construction.

The preliminary results appear to show that the reduced zoning requirements have resulted in more ADUs in these cities. For example, since adopting its ADU program in 2009, Vancouver has permitted over 3,000 ADUs and has set the target of adding another 4,000 by 2028. Recent regulatory reform, fee waivers, and public education efforts in Portland have contributed to the doubling of the number of annual ADUs permits, from under 300 in 2014 to over 600 in 2016.

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Last Modified: April 02, 2021