Encouraging Backyard Cottages

What is an ADU?

Accessory dwelling units (ADUs) are small, secondary dwellings allowed in residential zones:

  • A detached accessory dwelling unit (DADU), often called a backyard cottage or carriage house, is an ADU located in a separate structure from the principal dwelling unit (i.e., the main house).
  • An attached accessory dwelling unit (AADU), often called an in-law apartment or basement suite, is an ADU located within or attached to the main house. 

What's Happening Now?

This summer, we are preparing to launch a gallery of our pre-approved DADU designs. Drawing from more than 160 design submissions, our selected plans will offer a range of DADU sizes and styles that accommodate different household needs and help Seattle residents get a permit to build a backyard cottage more quickly and easily. We are currently working with the designers of the selected plans on the pre-approval process and will update this site and our email list as soon as they are available.

Want to learn more?

On July 9, 2019, Mayor Jenny Durkan signed legislation to remove regulatory barriers and make it easier for property owners to create accessory dwelling units (ADUs) in Seattle's single-family zones. The new ADU regulations took effect on August 8, 2019.

Interested in creating an ADU? Start with SDCI's updated tip sheets:

In addition to signing the ADU legislation in July 2019, Mayor Durkan also issued an Executive Order to implement several additional strategies to help more people be able to own and rent ADUs, including:

  1. Assisting homeowners in the ADU design and permitting process
  2. Piloting strategies to support equitable ADU development and affordability for homeowners and tenants
  3. Convening a workgroup to advise on City ADU programs and services
  4. Monitoring ADU production continually to evaluate the benefits and effects 

Project Benefits

Encouraging ADUs can increase the supply and variety of housing options in single-family neighborhoods. Backyard cottages share many characteristics with small single-family houses: a single unit with no shared walls in quiet residential neighborhoods. Due to their smaller size and lack of additional land cost, ADUs can offer a more affordable housing option in neighborhoods where homes are out of reach to many people.

ADUs can support the housing needs of a wide range of households, particularly older adults looking to downsize, seniors with fixed incomes, lower- and middle-income households, homeowners with disabilities or other needs, and multigenerational families. Households can benefit from the flexibility, stability, or supplemental income that an ADU provides in order to meet their evolving household needs.

End Result

Removing regulatory barriers will help property owners permit and construct ADUs, broadening the mix of housing options available in single-family zones.


Sign up for email updates on the progress of our work and opportunities for public comment.

Tell Us Your Story

Have you built or considered building an ADU? Do you have ideas for how the City can help more people create, rent, and live in ADUs? We would love to hear your experience. Please contact Nick Welch at nicolas.welch@seattle.gov to share your story.

Community Meetings

In January and February 2016, we co-hosted two community meetings with Councilmember Mike O'Brien to discuss policy options that could remove barriers to creating backyard cottages and ADUs.

City Council Meetings

Final Environmental Impact Statement

On October 4, 2018, the City issued a Final Environmental Impact Statement (EIS) analyzing potential environmental impacts of Land Use Code changes related to ADUs. Visit the ADU EIS website to read the Final EIS. The hearing for this appeal concluded on March 29, 2019. In May 2019, the City's Hearing Examiner upheld the adequacy of our EIS.

SEPA Draft Legislation

In May 2016, Councilmember Mike O'Brien published draft legislation for environmental review under the State Environmental Policy Act (SEPA), along with the following materials:



  • The City of Seattle has allowed attached ADUs and detached ADUs in all single-family zones since 1994 and 2010, respectively.
  • To encourage more of this housing type, in 2014 the City Council adopted Resolution 31547 directing us to explore changes that could make them easier to build and allow them on more lots in Seattle.
  • In May 2019, the City's Hearing Examiner upheld our environmental review of the proposal, clearing the way for legislative action on a proposed ordinance.
  • In July 2019, the Council adopted and Mayor Durkan signed legislation removing regulatory barriers to ADU production. The Mayor also issued an Executive Order calling for additional strategies to encourage ADUs for all.

Reference Documents