5 county-level rules, plus city-specific rules for 4 cities in Snohomish County, Washington.
Verified from official government sources
In unincorporated Snohomish County an accessory dwelling unit may not exceed 1,200 square feet of floor area and must sit on the same lot as the principal single-family dwelling. Urban lots may have up to two ADUs.
SCC 30.28.010
The floor area of an accessory dwelling unit shall not exceed 1,200 square feet. Floor areas shall be exclusive of garages, porches, unheated storage areas, and unfinished basements.
In unincorporated Snohomish County a one-story detached tool or storage shed no larger than 200 square feet with sidewalls up to 10 feet needs no building permit, but it must sit 20 feet from other buildings and property lines.
SCC 30.50.103
One-story detached accessory structures not used for human habitation, including tool and storage sheds, are exempt from permits when the floor area does not exceed 200 square feet and the sidewalls do not exceed 10 feet in height measured from grade average.
Converting a garage into habitable space in unincorporated Snohomish County is treated as creating an accessory dwelling unit and must meet SCC 30.28.010 ADU standards, including the 1,200-square-foot cap, water/sewer capacity, and off-street parking.
SCC 30.28.010
Accessory dwelling units are permitted uses in the urban zones pursuant to SCC 30.22.100 on lots with a single family, single family attached, or duplex dwelling. The floor area of an accessory dwelling unit shall not exceed 1,200 square feet.
In unincorporated Snohomish County a carport is an accessory structure used to cover vehicles or boats. Detached garages and storage structures over 2,400 square feet must sit at least 15 feet from external property lines; over 4,000 square feet, at least 20 feet.
SCC 30.23.110
Detached accessory or non-accessory storage structures and private garages with building footprints over 2,400 square feet must be at least 15 feet from any external property line; provided, that parcels abutting open space tracts shall have a five-foot setback from the open space.
In unincorporated Snohomish County a recreational vehicle cannot be a primary residence. A tiny house counts as a dwelling only if built on a foundation, permitted, and connected to water and sewer; a tiny house on wheels is regulated as an RV.
SCC 30.91R.050
"Recreational vehicle (RV)" means a travel trailer, motor home, truck camper, or camping trailer that is permanently designed and used as temporary living quarters, is either self-propelled or mounted on or drawn by another vehicle, is transient, and is not immobilized or permanently affixed to a mobile home lot.
4 cities in Snohomish County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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