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

In September, 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.

“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.

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.

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.