Seattle permits attached (AADU) and detached (DADU) accessory dwelling units ministerially under SMC 23.42.054 and 23.44.041, with HB 1337 (2023) requiring up to two ADUs per residential lot statewide. Applications are filed with Seattle Department of Construction & Inspections (SDCI) and SDCI must act within 120 days (Type I land-use review).
SMC 23.42.054 (AADU/DADU standards) and SMC 23.44.041 govern ADU permitting. Up to two ADUs are allowed per lot in Neighborhood Residential and Lowrise zones (one attached, one detached, or two detached on lots qualifying under HB 1337). Applications use SDCI's online Project Portal at seattle.gov/sdci. Review is ministerial (Type I) with no design review and no public notice required. SDCI publishes pre-approved DADU plan sets that cut review time to about 30 days. Required components: site plan, building plans, energy code compliance (Seattle Energy Code), structural plans, sewer/water capacity confirmation from SPU. Maximum DADU height is 18 feet for 1-story or 23 feet for 2-story (24 ft if green-roof, SMC 23.44.041.B.2). Maximum gross floor area is 1,000 sq ft.
Building an ADU without permits triggers an SDCI Notice of Violation under SMC 23.90, daily civil penalties of $150-$500, and double permit fees on application to legalize. Continued non-compliance can result in Director's Orders to remove or restore the structure.
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