How many ADUs can I build on my Seattle lot?
✓ Verified against code · August 19, 2026
Two, in addition to your house — attached, detached, or both, and two detached units may stack in a single structure. Seattle has allowed two ADUs since 2019.
Citation: SMC 23.42.022.C–.D
How current is this?
Seattle’s ADU rules run under SMC 23.42.022 and the new Chapter 23.44 (Neighborhood Residential) (Ordinance 127376 (CB 120993)). Seattle rewrote its residential zoning twice in eight months: the HB 1337 compliance package (Ord 127211, May 2025), then the permanent state-compliance code signed December 22, 2025 and effective January 21, 2026, which repealed the old Chapter 23.44 and amended SMC 23.42.022. Most guides written in 2025 describe interim rules that no longer exist. We verified this answer against primary sources on August 19, 2026, and every change we catch is logged in the changelog.
More Seattle ADU questions
- Does a backyard cottage count against Seattle's floor-area limits?
- Do I need to add parking for an ADU in Seattle?
- Do I have to live on the property?
- Can I sell a Seattle ADU separately from the house?
- What's the fastest way to permit a DADU in Seattle?
All Seattle ADU rules → · Compare Washington cities →
Not legal advice — lot-specific overlays can change the answer for your parcel. The citation above points at the governing code section; check it, or ask the city to confirm.