Accessory Structures in Seattle, WA (2026)
9 verified accessory structures rules for Seattle, Washington, sourced directly from the municipal code and official government pages.
Verified from official government sources
ADU Rules
Seattle adopted Ordinance 127211 (June 2025) to comply with HB 1337. Up to 2 ADUs allowed per lot by right in all residential zones (SMC 23.42.022). No owner-occupancy requirement. Max ADU size 1,000 sq ft. ADUs cannot be used as STRs. DADUs allowed up to 32 ft in most NR zones.
Seattle ADU & DADU Rules
Few RestrictionsADU Permits
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).
Seattle ADU Permit Process
Few RestrictionsADU Impact Fees
Seattle does not charge transportation, school, or park impact fees on ADUs because the city has not adopted general impact fees under RCW 82.02. Permit fees, sewer/water capacity charges from Seattle Public Utilities, and Mandatory Housing Affordability (MHA) fees may apply to larger projects, but standalone ADUs are largely exempt.
Seattle ADU Impact Fees
Few RestrictionsADU Owner Occupancy
Seattle eliminated all owner-occupancy requirements for ADUs in 2019 under Ordinance 125854, and this is now reinforced by Washington HB 1337 (codified at RCW 36.70A.696) which prohibits cities from imposing owner-occupancy on ADUs statewide. SMC 23.42.054.B.4 explicitly states no owner-occupancy is required.
Seattle ADU Owner Occupancy
Few RestrictionsADU Rental Restrictions
Seattle limits short-term rentals (under 30 days) of ADUs to two units per operator under SMC 6.600 — typically the operator's primary residence plus one additional dwelling. ADUs rented short-term must hold a Seattle short-term rental operator license and the rental-platform license. Long-term ADU rentals require RRIO registration under SMC 22.214 but have no minimum term.
Seattle ADU Rental Restrictions
Some RestrictionsShed Rules
One-story detached accessory structures (sheds, playhouses) under 200 sq ft do not require a building permit under Washington State Building Code (IRC). Structures in Environmentally Critical Areas require additional ECA review regardless of size. Setback and zoning requirements apply.
Seattle Shed Rules
Few RestrictionsGarage Conversions
Garage conversions to ADUs are allowed in Seattle under SMC 23.42.022 as part of HB 1337 compliance. Conversion provisions apply in all residential zones. Building permits required from SDCI. Must meet Seattle Building Code and accessibility standards.
Seattle Garage Conversion Rules
Few RestrictionsCarport Rules
Carports in Seattle's Neighborhood Residential zones are regulated as parking/garage accessory structures under SMC 23.44.016, with placement controlled by SMC 23.44.014 (yards). They cannot occupy the front 20 feet of a lot or side yards within 5 feet, and the combined footprint counts toward the 35% lot-coverage limit on lots 5,000 sq ft or larger.
Seattle Carport Rules
Some RestrictionsTiny Homes
Seattle allows tiny houses on permanent foundations as Detached Accessory Dwelling Units (DADUs) under SMC 23.42.022 and SMC 23.44.041, subject to the same code as any DADU. Tiny houses on wheels (THOWs) are treated as recreational vehicles and may not be used as residences in city residential zones. Sanctioned tiny house villages exist only as transitional encampments under SMC 23.42.054.
Seattle Tiny Home Rules
Some RestrictionsSeattle Municipal Code 23.42.022 — Accessory Dwelling Units (Tiny houses on wheels vs. DADUs)
Accessory Dwelling Unit — An accessory dwelling unit (ADU) is a separate living space within a house or on the same property as an existing or proposed house. These units may not be subdivided from the main house and are not legal unless they have been established through a permit process. A legally permitted unit in the home is called an attached accessory dwelling unit (AADU). A legally permi...
Looking for King County county-wide rules?
County ordinances apply to unincorporated areas and may supplement Seattle city rules.
Accessory Structures in King County →