skip navigation
MRSC logo

Have a Research Question?

Ask MRSC

Have a Research Question?

Ask MRSC

Accessory Dwelling Units (ADUs)

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

New Legislation: Effective June 6, 2024, HB 2375 extends the senior citizen and disabled persons property tax exemption and qualification for deferral programs to one detached accessory dwelling unit.

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.

Sketch image of attached ADUs compared to detached ADUs

Statutory Requirements for ADUs in Washington State

Since HB 1337 became effective in July 2023, state law (RCW 36.70A.680 and 36.70A.681) requires all local governments planning under the Growth Management Act (GMA), regardless of population and including counties (unincorporated urban growth areas) as well as cities, to revise their regulations as needed to conform, within six months after their periodic update due date, with the following requirements:

  • Minimum number of ADUs per lot: Two ADUs per lot must be allowed in all GMA urban growth areas, in addition to the principal unit, for lots that meet the minimum lot size required for the principal housing unit. Local regulations must permit ADUs to be attached, detached or a combination of both types. In addition, a conversion of an existing structure, such as a detached garage, must be allowed.
  • Maximum ADU size standard: Local governments may not require ADUs to be smaller than 1,000 gross square feet in size.
  • Dimensional standards: A local government may not impose setback requirements, yard coverage limits, tree retention mandates, or restrictions on entry door location that are more restrictive than those required for the principal unit.
  • Street improvements: A local government may not require street improvements as a condition of permitting accessory dwelling units.
  • Owner occupancy: A local government may not require owner occupancy for a principal unit or ADUs.
  • Condominium sales: Local governments may not prohibit the sale or other conveyance of a condominium unit independently of a principal unit solely on the grounds that the condominium unit was originally built as an ADU.
  • Design review: Local governments may not impose aesthetic standards or requirements for design review that are more restrictive for ADUs than those for principal units.
  • Required parking: There are restrictions on how much on-site parking can be required, with a sliding scale for smaller-sized lots. No on-site parking standards may be applied to ADUs located within a half-mile of a major transit stop.
  • Impact fees: Impact fees for ADUs are limited to no more than 50% of those assessed to the principal housing unit.
  • Common Interest Communities: New “Common Interest Communities” (for example, a new subdivision with a homeowners association) are prohibited from adopting covenants, conditions, and restrictions (commonly called “CC&Rs”) that would limit the construction of ADUs on any lot. Existing CC&Rs, however, are not impacted by the new law and may remain in effect.

Examples of ADU Codes in Washington State

Below are examples of selected ADU codes and ordinances from Washington State that contain code sections that comply with different aspects of HB 1337 as described in the example annotations.

For detailed guidance on compliance with all statutory requirements, including more code examples, see Commerce’s Guidance for Accessory Dwelling Units in Washington State (2023).

Note that local governments planning under the GMA, will revise their regulations as needed to fully conform to the 2023 state requirements within six months after their periodic update due date, as discussed in the above section on statutory requirements. As GMA planning cities and counties update their codes to be fully compliant, we will be posting and noting those examples on this page.

Examples of ADU Programs Outside Washington State

Below are examples of innovative ADU programs outside of Washington State that have been successful at expanding ADU development by focusing on removing zoning and permitting barriers, reducing fees/waivers for ADUs, or other strategies.

Short-Term Rental of ADUs and Enforcement

In recent years, ADUs have been increasingly used as short-term rentals (STRs) on online platforms such as Airbnb and VRBO. This use of ADUs and other types of housing, has a negative impact on the affordable housing supply in cities (see MRSC's blog post on Affordable Housing and the Impact of Short-Term Rentals). Some cities, such as Poulsbo, do not permit the use of ADUs as STRs. Others, such as Langley, limit the amount of STR permits approved for ADUs.

Enforcing STR regulations is challenging--it is very difficult for cities to determine how many or which ADUs are being offered as STRs on online rental platforms. As such, technology is playing a growing role in local enforcement of STR rules. Many cities across the U.S., including Madison (WI) and Charleston (SC), are starting to use 3rd party technology companies to monitor and report on STRs. Portland (OR) is in the process of developing its own data analyzing and enforcement software (see Portland's page on Short-Term Rental Regulation). Some large cities, such as Seattle and Los Angeles (CA), have had the benefit of collaborating with Airbnb on enforcement (see Los Angeles Magazine article (2020)). Despite these techniques, the enforcement of STR regulations remains a significant challenge for local governments of all sizes.

Examples

Recommended Resources

 

Last Modified: July 17, 2024