ADU (short for accessory dwelling unit) requirements vary by municipality. In Western Washington, most cities allow both attached (AADU) and detached (DADU) units. That said, no two cities are alike.

To help you understand what you can build in your neighborhood, we’ve compiled the core rules that apply to ADUs in Seattle and its surrounding cities. All information listed here is courtesy of The Master Builder’s Association of King & Snohomish County and is current as of January 2020. 

Remember that this list is just a starting point. Click the link on each city’s name to read more about its ADU codes, or give us a call. We can review the specifics and discuss what’s possible. 

Arlington

  • One AADU or DADU allowed. 
  • An ADU can’t be established on any parcel smaller than 4,200 square feet. 
  • The gross floor area (calculated from finished wall to finished wall) of an existing structure, an addition, or new detached structure, converted to or constructed for the purpose of creating an ADU, shall not exceed 30% of the gross floor area of the primary single family structure (not including garage and/or detached accessory buildings) or 800 square feet (whichever is less).
  • The total square footage of impervious surfaces on the lot (lot coverage) cannot exceed 35%. 
  • One additional parking space must be provided and owner occupancy is required in one of the two units. 

Everett

  • One AADU or DADU allowed. 
  • The ADU must not exceed 75% of gross area of the principal unit or 800 square feet, whichever is less. 
  • Most have design standards requiring consistency with the primary residence. 
  • Requires additional off-street parking stall unless within a quarter mile of a transit stop. 
  • Owner occupancy is required in one of the two units. 

Federal Way

  • One AADU or DADU allowed. 
  • The ADU must be between 300–800 square feet, but no more than 40% of the square footage of the primary dwelling unit. 
  • The ADU can’t have more than two bedrooms. Maximum lot coverage and parking requirements based on RS zone. 
  • One of the dwelling units must be owner-occupied for at least six months a year. 

Issaquah

  • One AADU or DADU allowed. 
  • The ADU must not exceed 1,000 square feet. 
  • One additional parking space required (tandem parking may be used to fulfill this requirement). The primary dwelling unit or the ADU must be occupied by an owner of the property or a family member of the property owner. 

Kenmore (in the process of updating code)

  • One AADU allowed. 
  • No DADUs. 
  • AADU allowed on a lot that is less than 10,000 square feet in area, or on a lot containing more than one primary dwelling. 
  • Off-street parking required. 
  • Owner occupancy is required in one of the two units. 

Kent

  • One AADU or DADU allowed. 
  • DADUs must not exceed 800 square feet or 33% of the size of the principal unit, whichever is smaller. 
  • AADUs must not exceed 40% of the principal unit. One off-street parking space is required. One of the units must be owner-occupied for at least six months a year. 

Kirkland (in the process of updating code)

  • One AADU or DADU allowed. 
  • An AADU cannot exceed 40% of the total area of the principal structure and the AADU. The size of a DADU may not exceed 800 square feet. 
  • The total area of all DADUs may not exceed 1,200 square feet plus 10% of the lot area that exceeds 7,200 square feet. 
  • The number of residents of the ADU and the principal dwelling unit combined must not exceed five unrelated individuals. 
  • One off-street parking space is required. 
  • Owner occupancy is required in one of the two units. 

Lake Stevens

  • One AADU or DADU allowed. 
  • Size must not exceed 50% of gross primary floor area or 800 square feet, whichever is less. 
  • One additional off-street parking space is required, in addition to existing SFR requirement. 
  • Owner occupancy is required in one of the two units. 

Lynnwood

  • One AADU or DADU allowed. 
  • Size must not exceed 800 square feet or 40% of the primary unit, whichever is less. 
  • A maximum of one bedroom shall be provided for units less than 600 square feet in size; a maximum of two bedrooms shall be provided for units 600 square feet or greater in size. 
  • One off-street parking space required. Owner occupancy is required in one of the two units. 

Marysville

  • One AADU or DADU allowed. 
  • The floor area of the accessory dwelling unit shall not exceed 35 percent of the total floor area of the single-family dwelling. 
  • Additional off-street parking spot required. 
  • Owner occupancy required (temporary, one-year exemption allowed in some cases) 

Mercer Island

  • One AADU or DADU allowed. 
  • The size of the ADU must be a minimum of 220 square feet and a maximum of 900 square feet, excluding any garage area. 
  • The square footage of the ADU shall not exceed 80% of the total square footage of the primary dwelling unit. 
  • Parking requirements depend on total square footage; see code. 
  • Primary dwelling unit or ADU must be occupied by an owner of the property or an immediate family member of the property owner. 

Mill Creek

  • One AADU or DADU allowed. 
  • ADUs must less than 400 square feet, but no more than 800 square feet, and in any event not more than 30% of the gross floor area of the residence. 
  • One additional parking space and owner occupancy are required. 

Mukilteo

  • One AADU or DADU allowed. 
  • AADUs are allowed on single-family residential lots 5,000 square feet or greater in size. 
  • DADUs are only permitted on lots 10,000 square feet or greater. 
  • The ADU must be not exceed 700 square feet or 60% of the primary dwelling unit size, whichever is smaller. 
  • No more than one bedroom. 
  • Four off-street parking spaces must be provided—two for the primary dwelling and two for the ADU. 
  • The parking spaces for each unit must be independent of the other spaces and must be accessible to the street without having to cross the other unit’s spaces. 
  • Owner occupancy is required in one of the two units. 

Newcastle

  • One AADU or DADU allowed. 
  • The ADU must be a minimum of 300 square feet and not more than 1,000 square feet in size and can’t exceed 40% of the total square footage of the primary dwelling unit. 
  • If the accessory dwelling unit is located completely on a single floor, the director of community development may allow the AADU to exceed these square footage requirements to the extent necessary to efficiently use all floor area. 
  • One additional parking space is required, for a total of at least three parking spaces. Owner occupancy is required in one of the two units. 

North Bend

  • One AADU or DADU allowed. 
  • ADUs must be located in the side yard or rear yard areas only. ADUs are not permitted on lots under 4,000 square feet and cannot be larger than 10% of the lot area or 800 square feet, whichever is smaller, and is limited to a maximum of one bedroom. 
  • Studios are allowed on lots under 5,000 square feet. 
  • ADUs must meet the height requirements of 1.51(b). 
  • Owner occupancy is required in the primary dwelling unit. 

Renton

  • One AADU or DADU allowed. 
  • The AADU must not exceed 1,500 square feet. 
  • If the AADU occupies an entire single floor, the Technical Committee may allow for an increase in the allowed size of the AADU in order to efficiently use all floor area. 
  • The DADU must not exceed 40% of the total square footage of the primary dwelling unit, and in no case can the DADU exceed 1,000 square feet. 
  • Property owners who propose an ADU must notify their adjacent neighbors of the project in writing. 
  • One off-street parking space is required in addition to the two parking spaces required for the primary dwelling unit. 
  • Owner occupancy is required in one of the two units. 

Renton (in the process of updating code)

  • One DADU allowed. 
  • No AADUs allowed. 
  • ADUs must not exceed 75% of the primary residence or 800 square feet, whichever is smaller. 
  • Recent policy change to incentivize ADUs in new construction: In a subdivision of 10 or more new residential units, development fees for ADUs are waived by 50% and waived in full for every third new ADU. 
  • Owner occupancy is required in one of the two units. 

Sammamish

  • One AADU or DADU allowed. 
  • The ADUs must not exceed a floor area of 1,000 square feet when detached, except when one of the dwelling units is wholly contained within the existing residence, in which case the floor area must not exceed 50% of the existing unit. 
  • No additional off-street parking space is required when the parcel contains four or more parking spaces. 
  • Owner occupancy is required in one of the two units. 

SeaTac

  • One AADU or DADU allowed. 
  • All ADUs need to be at least 220 square feet in area. 
  • The maximum size of the ADU will vary depending upon the type built. 
  • DADUs, ADUs created through an addition, and those designed into the structure at the time of permitting can be up to 800 square feet. 
  • An ADU created within an existing structure can be up to 45% of the total area of the primary unit. 
  • One off-street parking space is required. 
  • For a minimum of nine months in any 12-month period, the owner of the primary unit must occupy one of the two units. 

Seattle 

  • Two ADUs allowed. 
  • It can be either two AADUs or one AADU and one DADU. 
  • The second unit must meet one of following requirements: 
    • Green building standard 
    • Affordable for the next 50 years
  • The ADU/DADU can be up to 1,000 square feet in single-family zones. 
  • There is no parking or owner occupancy requirement. 

Shoreline

  • One AADU or DADU allowed. 
  • The ADU cannot exceed 50% of the size of the main dwelling. 
    • Exception: An ADU interior to the residence may be larger than 50% of the primary residence where the unit is located on a separate floor and shares a common roof (i.e. daylight basement). One off-street parking space is required in addition to two for the main house. 
  • This requirement may be met using tandem (end-to-end) spaces for the minimum parking for the main house. 
  • The owner of the property or an immediate family member must live in either the main house or the ADU. 

Snohomish

  • One AADU or DADU allowed. 
  • The ADU cannot exceed 50% of gross primary floor area or 800 square feet, whichever is less. It must comply with city design standards. 
  • One additional parking space is required, for a total of three parking spaces. 
  • Owner occupancy is required in one of the two units. 

Snoqualmie (in the process of updating code)

  • Has a Residential 2 (R2) zone that allows for the construction of duplex, triplex, quadruplex, attached townhomes of no more than four units, and small AADUs/DADUs. 

Stanwood

  • One AADU or DADU allowed. 
  • ADUs cannot exceed 50% of primary residence or 900 square feet, whichever is less. 
  • Design elements must be consistent with main house. 
  • One additional parking space is required. 
  • Owner occupancy is required in one of the two units. 

Tukwila

  • One AADU or DADU allowed. 
  • DADUs must be on a minimum lot size of 6,500 square feet and the unit may be up to 800 square feet. 
  • AADUs may occupy 40% of the square footage of the house, up to a maximum of 1,000 square feet, whichever is less. 
  • One additional parking space is required. 
  • Owner occupancy is required in one of the two units. 

Unincorporated King County

  • One AADU or DADU allowed. 
  • If the urban lot is less than 5,000 square feet in area or has more than one primary dwelling, only an AADU is allowed. 
  • One of the dwelling units shall not exceed a floor area of 1,000 square feet, except when one of the dwelling units is wholly contained within a basement or attic. 
  • One additional parking space is required. 
  • Owner occupancy is required in one of the two units. 

Unincorporated Snohomish County

  • One AADU or DADU allowed. 
  • One additional parking space is required. 
  • Owner occupancy is required in one of the two units. 
  • For DADUs: The minimum floor area must be 360 square feet and not exceed 40% of the floor area of the single-family dwelling unit or 850 square feet, whichever is less. 
  • For AADUs: If the floor area of the single-family dwelling unit is under 2,000 square feet, the floor area of the AADU shall not exceed 40%. 
  • If the floor area of the single-family dwelling unit is under 2,000 square feet or more, but less than 3,000 square feet, the floor area of the AADU shall not exceed 35% or 800 square feet, whichever is greater. 
  • If the floor area of the single-family dwelling unit is under 3,000 square feet or more, but less than 5,000 square feet, the floor area of the attached accessory apartment shall not exceed 30% or 1,050 square feet, whichever is greater. 
  • If the floor area of the single-family dwelling unit is over 5,000 square feet, the floor area of the AADU can’t exceed 20% or 1,500 square feet, whichever is greater. 

Woodinville

  • One AADU or DADU allowed. 
  • DADU may not be larger than 50% of the living area of the primary residence. 
  • One additional parking space is required. 
  • Owner occupancy is required in one of the two units.