Snohomish County, WA Accessory Structures: ADU Rules (2026)

SCC 30.28.010(1) requires every ADU to sit on the same lot as a legally-established principal dwelling and to comply with all other Title 30 provisions. SCC 30.28.010(2) sets the maximum ADU floor area at 1,200 square feet, exclusive of garages, porches, unheated storage areas, and unfinished basements. SCC 30.28.010 permits a maximum of two ADUs on lots whose principal unit is a single-family dwelling - either one attached plus one detached, or two detached - and one ADU on lots with a single-family attached (townhome) principal unit. Setbacks from public roads for ADUs are reduced to five feet on roads 60 feet wide and over, and on roads under 60 feet in a recorded subdivision, short subdivision, or binding site plan, measured from the right-of-way edge. ADUs must meet the off-street parking standards of SCC chapter 30.26, and applicants must document that the existing or proposed sewage/septic system can handle the additional load and that water supply is potable and of adequate flow. RCW 36.70A.681 (HB 1337, 2023) overlays statewide minimums: counties planning under the Growth Management Act (Snohomish is one) must allow at least two ADUs per lot in unincorporated UGAs zoned for single-family use, may not impose owner-occupancy requirements, must permit ADUs to be sold as condominium units under RCW 64.32 or 64.34, and cannot require more than one off-street parking space per ADU on lots smaller than 6,000 sq ft (none within a half-mile of major transit). Snohomish County Planning and Development Services issues building permits through MyBuildingPermit.com.