Encouraging Backyard Cottages

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What & Why

Background

Changes to ADU policy to fulfill state requirements

In May 2025, the City Council adopted Ordinance 127211, which makes changes to Seattle's rules for ADUs to fulfill requirements in House Bill 1337. In June 2025, Council adopted Ordinance 127219, adopting interim changes to Seattle's Neighborhood Residential (NR) zoning to fulfill requirements in House Bill 1110 to allow middle housing citywide. These changes took effect on June 30, 2025. The Council has also advanced permanent HB 1110 legislation, which will include further changes to standards for housing in NR zones and is likely to be adopted later in 2025. Follow this process on the City Council Comprehensive Plan website.

ADUniverse

ADUniverse launched in September 2020 with a gallery of pre-approved DADU designs, a step-by-step guide to the ADU process, and a search tool to identify the feasibility of adding an ADU to your property. As we update the information on ADUniverse to reflect recently implemented state requirements for ADUs and middle housing, some sections of ADUniverse may be out of date.

Pre-approved DADU plans

View our gallery of pre-approved designs, created by local designers and architects. These building plans offer a faster, easier, and more predictable permitting process for creating a DADU. In most cases, you can get your permit in just 2-6 weeks. For background on this initiative, read our submission guide, design criteria survey, and one-pager.

Interested in creating an ADU? 

Start with SDCI's updated tip sheets:

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.

Reports

Timeline

  • 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.
  • In September 2020, we launched ADUniverse, a new resource for Seattle residents that features our pre-approved plans, a step-by-step guide for prospective ADU owners, and a search tool to help homeowners assess the feasibility of adding an ADU on their property.
  • In May 2025, the City Council adopted Ordinance 127211, which makes changes to Seattle's rules for ADUs to fulfill requirements in House Bill 1337.

2019 ADU Legislation

In July 2019, the City adopted 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.

Environmental Impact Statement

SEPA draft legislation

Planning and Community Development

Rico Quirindongo, Director
Mailing Address: P.O. Box 94788, Seattle, WA, 98124-7088
Phone: (206) 386-1010
opcd@seattle.gov

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The Office of Planning and Community Development (OPCD) develops policies and plans for an equitable and sustainable future. We partner with neighborhoods, businesses, agencies and others to bring about positive change and coordinate investments for our Seattle communities.