ADU rules in Snohomish County, WA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
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.
Under SCC 30.28.010, ADUs are permitted where a single-family, single-family-attached, or duplex dwelling exists. Floor area is capped at 1,200 square feet, excluding garages, porches, unheated storage, and unfinished basements. In urban zones a lot may contain up to two ADUs (one attached plus one detached, or two detached). ADUs must have physical and legal access to potable water and adequate sewer/septic capacity, and must meet off-street parking under chapter 30.26 SCC. Standards differ in rural and resource zones, and cities within the county set their own ADU rules.
Building/using an ADU without required permits is a code violation handled by Planning & Development Services; stop-work and correction orders plus daily civil penalties may apply until permits are obtained.
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