DADU Regulations | Seattle

• In Seattle, both attached (AADU) and detached (DADU) accessory dwelling units are permitted on lots where single-family dwellings are allowed. Seattle+2Seattle+2

• Number of units per lot: Up to two ADUs (in addition to the primary dwelling) are allowed on one lot. These can be any combination of attached and detached units. Seattle+2Building Connections+2

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• Maximum size:

  • The gross floor area of an ADU may not exceed 1,000 square feet.

  • For a DADU in many zones the same 1,000 sq ft limit applies.

    
    

• Minimum lot size for a DADU: A lot must be at least 3,200 square feet (in neighborhood residential zones) to build a detached unit.

  
  

• Parking requirement: No additional off-street parking is required for ADUs or DADUs. Existing required parking spaces should not be removed unless a replacement compliant with code is provided.

  
  

• Owner-occupancy requirement: Seattle no longer requires the property owner to live in the principal dwelling or the ADU/DADU.

  
  

• Height and other dimensional standards: The height limit for the ADU is generally the same as that for the primary dwelling in the zone. Setback and other dimensional standards follow those for the principal home rather than being more restrictive.

  
  

• Other notes: Some spaces are excluded from the size cap: up to 250 sq ft of an attached garage, exterior‐only accessed storage, underground stories, and up to 35 sq ft of long‐term bicycle parking.