House Bill 1337 – Accessory Dwelling Units (ADUs) in Washington State

Why its a big deal

House Bill 1337 was passed in April of 2023, and it is a big deal for our state. The purpose is to ease the barriers to construction of accessory dwelling units (ADUs) in the state of Washington and to help solve the housing affordability crisis. ADUs are more affordable to build than full houses, and they also tend to rent for less. As the bill states, “The legislature intends to promote and encourage the creation of accessory dwelling units as a means to address the need for additional affordable housing options.”

The bill allows for greater urban density, and also creates opportunities for home owners. It will soon be easier than ever to build an accessory dwelling unit in a city residential lot. The most important parts of the bill are the ability to built up to two ADUs on a residential piece of property, and also the lifting of any residence requirement. The owner of the property does not have to live on the property that has an ADU. That means that people with rental properties can maximize their investment by adding dwelling units to their existing properties.

Required changes

The following list is a summary of the changes that are mandated by House Bill 1337. Please see the bill itself for the specific terminology and all details.

  • Impact fees on the construction of an ADU must be less than 50% of the impact fees that would be imposed on the principal unit.
  • Property owners¬† are not required to live on a property that has an ADU.
  • Lots that allow single-family homes must also allow for up to two ADUs.
  • ADUs must be allowed in structures that are detached from the main dwelling unit.
  • ADUs must be allowed on lots that meet the minimum lot size requirements for a residential unit.
  • The maximum allowable floor area for the ADU cannot be less than 1000 square feet.
  • The height requirement for the ADU cannot be less than the height requirement of the principal residence, or less than 24 feet high.
  • Requirements for setbacks, entry door placement, aesthetics, and other considerations can’t be more restrictive than the requirements for the principal residence.
  • ADUs can abut to the property line if it abuts to a public alley, unless the alley is plowed for snow.
  • Existing structures may be converted to ADUs, even if they violate current code requirements for setbacks and lot coverage.
  • The sale of a condominium cannot be restricted solely on the basis that it was built as an ADU.
  • Off-street parking cannot be a requirement to build and ADU if the unit is within half a mile of a major transit stop.
  • Only one off-street parking space can be required per ADU on properties under 6000 square feet.
  • Only two off-street parking spaces can be required per ADU on properties over 6000 square feet.
  • Cities may imposed a limit of 2 dwelling units on residential lots of 2000 square feet or less.
  • Cities may provide incentives for building ADUs, such as waving of fees or deferment of taxes, if the units are located in an urban growth area or otherwise built to further affordable housing.

When will the changes be adopted?

Cities have until six months after their next comprehensive plan update. This means that, according to RCW 36.70A.130, the changes will be adopted between 2024-27. Of course, cities are free to adopt the changes before this time and some are doing so.

Schedule of adoption by counties*

December 31, 2024
King, Kitsap, Pierce, and Snohomish

June 30, 2025
Clallam, Clark, Island, Jefferson, Lewis, Mason, San Juan, Skagit, Thurston, and Whatcom

June 30, 2026
Benton, Chelan, Cowlitz, Douglas, Franklin, Kittitas, Skamania, Spokane, Walla Walla, and Yakima counties

June 30, 2027
Adams, Asotin, Columbia, Ferry, Garfield, Grant, Grays Harbor, Klickitat, Lincoln, Okanogan, Pacific, Pend Oreille, Stevens, Wahkiakum, and Whitman

* The deadline for adoption is six months after these dates. 

Get started with your ADU

Are you ready to build an ADU? Mount Baker Tiny Homes is ready to assist you, whether you want to place a tiny home on wheels (THOW) as an ADU, or you want to build an ADU on a foundation. We build in our factory and then ship out to any location. We have the knowledge, expertise, and plans to make your ADU reality. Please contact us if you have any questions.


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