DADU Regulations | Snohomish County

• In Snohomish County urban zones (see SCC 30.22.100) accessory dwelling units are permitted on lots with a legally-established principal unit.

  
  

• Number of units per lot (urban zones):

  • If the principal unit is a single-family dwelling or duplex: up to two ADUs are allowed on the lot in the following configurations:

    • One attached + one detached ADU, or

    • Two attached ADUs, or

    • Two detached ADUs (in one or two structures).

  • If the principal unit is a single-family attached dwelling: one ADU is permitted (either attached or detached) on the lot.

    
    

• Maximum size: The floor area of an ADU shall not exceed 1,200 square feet, exclusive of unheated storage, garages, porches and unfinished basements.

  
  

• Bulk/dimensional incentives: For lots in urban zones:

  • Maximum lot coverage may be increased (to about 55 %) for properties with ADUs.

  • Setbacks from private roads reduced to five feet for principal units, ADUs and covered parking.

    
    

• Water & sewer/septic requirement: Developers must demonstrate that potable water is available and that the existing or proposed sewage or septic system is capable of handling the additional demand.

  
  

• Where detached ADUs are prohibited (rural/resource zones): Detached ADUs are prohibited on lots that do not meet the zone’s minimum lot area. For example: in the R-5 zone if lot is less than 5 acres; in RC zone if less than 100,000 sq ft.

  
  

• Recent updates: With Ordinance 25‑014 (March 19, 2025), Snohomish County revised ADU regulations to relax certain design standards, expand where ADUs are allowed (including duplexes and attached single-family dwellings in urban zones) and remove some prior architectural requirements.

Assistance Bulletin #12 | Detached Accessory Dwelling Units