Mayor Harrell Announces Proposal to Boost Production of Accessory Dwelling Units for More Diverse Housing Opportunities

New proposal goes above state requirements with incentives to create more family-sized homes, preserve existing homes, and expand homeownership opportunities

Seattle - Today, Mayor Bruce Harrell announced a package of new legislation to streamline the permitting process and increase production of accessory dwelling units (ADUs) in Seattle, creating more diverse and affordable housing opportunities.

ADUs are secondary dwellings located in residential zones. They include detached accessory dwelling units (DADUs), often called backyard cottages, and attached accessory dwelling units (AADUs), often called in-law apartments or basement suites.

"Making progress on Seattle's urgent housing affordability needs requires not only increasing overall production, but increasing the diversity of housing available," said Mayor Harrell. "ADUs are a key element of our One Seattle Housing Agenda as an efficient, sustainable option to address the housing shortage while also creating new opportunities for first-time homebuyers, multigenerational living, age-friendly housing, and financial stability for homeowners while also preventing displacement. Seattle's ADU ordinance is one of the most progressive in the country, and this forward-thinking proposal will enhance it to create more flexible living options and inclusive, vibrant communities throughout our city."

The new proposal includes legislation to update Seattle's existing ADU code in compliance with HB 1337 ahead of the state's June 2025 deadline. Building off existing code in Seattle's Neighborhood Residential Zones, this bill will standardize the ADU code citywide, allowing two ADUs per lot and dual detached accessory dwelling unit configurations across all residential zones. It also includes more flexible design standards for ADUs and exempts them from street improvement requirements, which can add significant additional costs.

In addition to adopting the provisions set out by the state, a companion bill will also be transmitted early next year following SEPA review to further incentivize the production of ADUs in Seattle. The companion legislation is expected to:

• Encourage more family-sized homes: Increases the maximum ADU size to 1,500 square feet to allow for the construction of 3-bedroom ADUs.

• Expand homeownership opportunities: Simplifies ADU sales by reducing the need to form an HOA.

• Preserve existing homes: Allows exceptions to certain design standards if the principal unit is retained and does not count ADU construction toward the substantial alteration threshold.

• Make it easier to fit a second ADU: Provides more building separation and attachment options, such as allowing AADU attachment via breezeways, which can offer more flexibility on unusually shaped lots.

"Our department remains committed to innovative policy changes that remove regulatory barriers and encourage a variety of housing types in Seattle," said Nathan Torgelson, Director of the Seattle Department of Construction and Inspections. "Demand for new housing continues to outpace the supply. Flexibility for new accessory dwelling units responds to statewide goals and creates additional housing opportunities for both new renters and owners in our city."

Since Seattle reformed its ADU rules in 2019 to make it easier for homeowners to build ADUs in residential areas, permitting has surged. A recent analysis from the Office of Planning and Community Development (OPCD) found that the City issued 987 permits for ADUs in 2023, a fourfold increase over 2018 permitting levels.

However, ADU permit applications have fallen this year by nearly 40% compared to the same period in 2023, as interest rates hit a 20-year peak. The new incentives will remove regulatory barriers to help make the construction of ADUs more feasible for homeowners, while remaining consistent with Seattle's building code, short-term rental code, and tree code.

Most ADU production is in Neighborhood Residential zones, offering diverse housing options in high-opportunity neighborhoods with high costs and limited production. ADUs sold as condos cost 40% less than a single-family home on the same parcel, and ADUs rent for about 25% less than the median one-bedroom apartment, according to a 2022 survey from OPCD. Approximately 12% of ADUs in Seattle are occupied by family or friends completely rent free.

Following SEPA review, the legislation to bring Seattle's ADU code in compliance with new state requirements will be transmitted to the City's Council's Land Use Committee and heard after the budget. The companion legislation is expected to be transmitted early next year.

Through an expansive One Seattle Housing Agenda, Mayor Harrell continues to drive progress to support more affordable housing and greater housing production in the city, including through the One Seattle Plan, which is expected to be submitted later this year. Other efforts include proposing a design review holiday for downtown developments, passing new incentives for office-to-residential conversions, advancing a new redevelopment plan for affordable housing at Fort Lawton, passing the largest housing levy in City history in 2023 which will build over 3,100 new affordable homes, adopting new State law to exempt residential and mixed-use projects from SEPA review, proposing legislation to permanently exempt affordable rental and homeownership projects from design review, and updating the City's maritime and industrial lands policy to create more housing opportunities.

What People Are Saying

Representative Mia Gregerson

"I want to extend my heartfelt thanks and congratulations to Mayor Harrell and his team for their continued leadership in updating policies that will help Seattle's residents and businesses build and plan for ADUs and DADUs. This effort is another significant step toward making our communities more age-friendly and inclusive. The stronger the partnership between state and local governments, the more efficiently we can close the housing supply gap and provide much-needed solutions for affordable housing. By working together, we can accelerate progress and ensure better outcomes for everyone."

Councilmember Tammy Morales, Land Use Committee Chair

"This legislation is an important step towards making Seattle a more affordable city. By streamlining the permitting process for ADUs and creating more family-sized, flexible housing options, we are giving homeowners the tools to contribute to a more diverse housing stock. I'm happy to bring this legislation forward as Chair of the Land Use Committee so we can expand access to affordable housing, homeownership, and intergenerational living opportunities while growing our toolkit to stem displacement across the City." 

Patience Malaba, Executive Director, Housing Development Consortium of Seattle-King County

"As a core member of the Black Home Initiative, the Housing Development Consortium is proud to support the City of Seattle's ADU reforms. This legislation builds on Seattle's existing, successful framework for ADUs, creating more flexibility to add homeownership options throughout all of Seattle's neighborhoods. It's a commonsense step to increase housing affordability and pathways to homeownership, in a way that strengthens neighborhoods and keeps families rooted in place."

Cathy MacCaul, Advocacy Director, AARP Washington

"AARP applauds the City of Seattle's efforts to ease barriers to build accessory dwelling units. By creating more affordable housing choices, older Americans will be able to remain in their homes and communities. These living arrangements will allow them to be closer to family caregivers and loved ones."

Erich Armbruster, President, Ashworth Homes

"Seattle is taking a critical step with this legislation by ensuring our city's ADU code aligns with state ADU law and provisions of Washington's middle housing bill. Equally important will be what comes next: lowering more barriers limiting our ability to build ADU's that meet the needs of more people. Whether built for growing families, parents or children, or older adults, ADUs can be a valuable resource in helping grow and keep communities together. I know Seattle is committed to finding ways to go above and beyond to unlock more housing opportunities for our neighbors. I look forward to partnering with Seattle on this mission that lies ahead."

Melissa K. Neher, Executive Director, AIA Seattle

"AIA Seattle envisions equitable, resilient thriving communities. AIA Seattle supports solutions that enhance housing capacity, allow greater flexibility of housing types, and foster a more diverse range of households at all income levels. The ADU legislation will offer more predictable development pathways as required by the Washington State Legislature, in turn expanding housing supply to the benefit of all members of our community. The updated ADU legislation provides clarity, which is an opportunity to create urgently needed housing options for all residents."

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