ADUs and Middle Housing Update

by SDCI Community Engagement on July 7, 2025

On June 30, 2025, several adopted pieces of legislation became effective in Seattle related to compliance with State laws aimed to boost housing supply and diversify housing options in Seattle’s residential zones. These code changes, put forward by Mayor Harrell and adopted by Council, are known as the HB1337 and HB1110 Compliance Bills (Ord. 127211 and 127219 respectively).

The changes to the Land Use Code create a flexible framework to boost housing supply that aligns with state goals to address Washington’s housing shortage, estimated at 140,000–250,000 units. These changes will diversify housing options and reduce barriers to constructing Accessory Dwelling Units (ADUs) and several other housing types, often referred to as Middle Housing. Together, these laws position Seattle to accommodate population growth, reduce sprawl, and enhance housing access.

ADUs

Seattle’s previously adopted ADU rules varied by zone. All general standards for ADUs are now consolidated in SMC 23.42.022 and are consistent between all zones that allow residential development. The new ADU standards also include more flexibility including:

• Allowing two ADUs per lot in addition to a primary unit on all lots.
• ADUs can be configured as one or more units in detached, attached, or stacked apartment flat configurations.
• Reduced requirements for an attachment between units (minimum 3 feet of attachment).
• Setback and height standards have been loosened to be no more restrictive than those for the primary home.
  • Additional height up to 32 feet in most Neighborhood Residenetial zones and higher in the Lowrise zones.
  • ADUs are allowed in the rear yard up to 32 feet.
• Size limits for ADUs have been raised to 1000 square feet across all zones where they are allowed.
• ADUs are exempt from street improvement requirements regardless of zone.

Middle Housing

The second ordinance, the HB1110 Compliance Ordinance, will adopt new rules to encourage several types of housing Citywide. This ordinance will allow up to 4 units on every residential lot and up to 6 units when the development provides low-income housing or is located near major transit stops. Code changes to the Neighborhood Residential and Lowrise code are extensive and include measures such as:

• Increased density. Existing lots, even those that are less than 5000 square feet, are allowed increased density of 4 units or 6 units if the lot provides low-income units or is near a major transit stop.
• A broader range of housing types in what has historically been single family zoning. These unit types include:
  • Detached units, such as stand-alone residences or cottage housing
  • Attached units, such as a townhouse or rowhouse developments
  • Stacked apartment flats
• Increased lot coverage allowances and reduced minimum setback requirements to accommodate more units on a lot.
• Height limit increases in all Neighborhood Residential and Lowrise zones.
• More flexible amenity area standards in Lowrise zones.
• Elimination of bicycle parking and bike parking storage requirements for all middle housing types.
• Increased requirements for Electric Vehicle (EV) ready parking.
  • Private parking spaces in a development need to be EV ready
  • The first 12 parking spaces in a parking area serving multiple units needs to be EV ready
  • Residential parking areas with more than 24 units need to provide EV ready stalls for 45% of the spaces

Our online Seattle Municipal Code may take several months to be updated to the new regulations and so a consolidated version of these codes is provided on the Office of Planning and Community Development’s website. If you have questions about these new rules, visit our website or submit your question to our Virtual Applicant Services Center. See our Opinion Letter page for a list of our paid land use information services for specific questions regarding the eligibility of an existing lot to accommodate additional density or to request more detailed zoning information.