ADU & DADU Regulations in Washington
DADU's and ADU's are an increasing popular project to increase housing density, improve home affordability, and provide a great return investment return for flippers and rental property investors. ADU's and DADU's are currently regulated at the city level - which means that each city has different rules about whether and what you can build on your lot. Below is a list of some cities that allow ADU's and DADU's rules, and a link to the full building code for more info.
DADU's are relatively new and growing in popularity quickly - although not all cities on this list currently allow them, as an investor you would be wise to purchase properties that have good DADU potential, even in cities that do not currently allow DADU's to be built. Here's what we at HouseHack Seattle look for in a property with good DADU potential:
1. Corner lots: This allows you to be creative with what is the "backyard" and what is the "front yard", which helps with placement of the DADU and setbacks. Corner lots also allow the main house to be accessed from one street, while the DADU is accessed from another street, increasing parking and privacy for the two units.
2. Lots with back alley access: Like corner lots, this is because of the setbacks and accessibility. Typically you need a 5 foot setback from the back lot-line, but if you have a back alley the setback requirement is removed and you can build right on the lot line! This results in more buildable area, larger yards for both the ADU and the DADU, more privacy, and better accessibility (if the back alley provides parking access for the DADU)
3. Level lots: As with all development projects, steep slopes tend to increase complexity and cost.
Here's an alphabetical list of Washington Cities & their ADU / DADU regulations:
Link to ADU Cheat Sheet: Zoning Codes, Permit Searches, and More!
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. The City of Arlington Public Works Department shall provide, at no charge, water, and sewer hookups for ADU.
arlingtonwa.gov/DocumentCenter/View/1270/Attachment-C
AUBURN
One AADU or DADU allowed. ADUs cannot exceed 50% of the primary unit or 950 square feet, whichever is less. Their style must match the primary residence. They cannot exceed 950 square feet or
be under 300 square feet or have more than two bedrooms. Only one ADU exterior entrance is allowed and it must be no closer than 10 feet to an adjoining property line. One additional parking space must be provided and owner occupancy is required in one of the two units.
auburn.municipal.codes/ACC/18.31.120
BELLEVUE
One AADU allowed. DADUs are not allowed. AADUs cannot exceed 40% of the total square feet of the residence. ADUs must be more than 300 square feet but not exceed 800 square feet. One off-street parking space is required. The combined number of occupants in the primary residence and the ADU may not exceed the number allowed by the definition of “family,” Land Use Code (LUC) 20.50.020. The City’s Development Services will provide notice of the registration of the ADU to owners of property within 500 feet of the site. Owner occupancy is required in one of the two units.
development.bellevuewa.gov/zoning-and-land-use/zoning-requirements/accessory-dwelling-units
BLACK DIAMOND
Two ADUs allowed. One AADU and one DADU allowed. One AADU is allowed within the existing building footprint and no more than one ADU resulting from new construction shall be allowed, including additions and garage or accessory structure conversions. DADUs cannot exceed 1,000 square feet. Parcels upon which an ADU has been approved shall not be subdivided or otherwise segregated in
ownership in a manner that would separate the ADU from the primary dwelling unit. One additional parking space must be provided, and owner occupancy is required in one of the two units.
blackdiamond.wa.us/citycode/level2/TIT18ZO_CH18.56ACDWUN.html
BOTHELL
One AADU or one DADU allowed. The floor area of the ADU can be no more than two-thirds of the primary dwelling unit, or 40% of the total floor area of the primary dwelling unit and ADU combined, excluding any garage area; or 800 square feet, whichever is less. DADUs are exempt from the provisions of code that limit the size of accessory buildings to 5% of the lot area. DADUs may not have a height greater than 25 feet or no greater than 28 feet for units over an existing accessory structure. Specific rules apply for two-story DADUs to protect the privacy of adjacent homes. One additional parking space must be provided, and owner occupancy is required in one of the two units.
bothell.municipal.codes/BMC/12.14.135
BURIEN*
Two ADUs allowed. ADUs may be one AADU and one DADU. Two DADUs not allowed. AADUs must not exceed 1,000 square feet. DADUs must not exceed 800 square feet. The director may make exceptions to size limitations to allow for the better utilization of existing spaces. Buildings must be at least five years old to be eligible for an exception to the size limitation. One parking space required; lots
within ¼ mile of a transit stop are exempt. Parking exemption will be reevaluated three years after ordinance goes into effect (ordinance passed Dec. 2019). No owner occupancy requirement.
burienwa.civicweb.net/document/31646/Adopt%20Ordinance%20No.%20724,%20Zoning%20Code%20Amendments.pdf
CARNATION
One AADU or DADU allowed. ADUs may not exceed 40% of the floor area of the primary dwelling unit. If the primary dwelling unit is less than 2,000 square feet in size, an ADU may not exceed 800 square
feet. Each ADU must be serviced by a pedestrian walkway connecting the adjacent street or alley to the primary entrance of the ADU. No more than two bedrooms shall be located within an ADU. One offstreet parking space must be provided. Owner occupancy is required in one of the two units.
library.municode.com/wa/carnation/codes/code_of_ordinances?nodeId=TIT15LAUS_CH15.46ACDW UN
COVINGTON
One AADU or DADU allowed. Only AADUs are allowed if the lot is less than 10,000 square feet. ADUs may not exceed 1,000 square feet except when one of the dwelling units is wholly contained within a basement or attic. One off-street parking space must be provided. Owner occupancy is required in one
of the two units.
codepublishing.com/WA/Covington/html/Covington18/Covington1825.html
DES MOINES
One AADU or DADU allowed. Referred to as “accessory living quarters (ALQ)” in Des Moines’ code. The ADU may not be less than 350 square feet of living space. ADU may not exceed 800 square feet if they are: (a) detached from the primary single-family dwelling; (b) created through an addition; or (c) designed into a new structure at the time of construction. For AADUs, the square footage limitation
may be no more than 35% of the total square footage of the primary dwelling unit. One off-street parking space must be provided. Owner occupancy is required in one of the two units.
codepublishing.com/WA/DesMoines/html/DesMoines18/DesMoines1855.html
DUVALL
One AADU or DADU allowed. A DADU shall not exceed 800 square feet of gross floor area. An AADU cannot exceed 1,500 square feet of gross floor area. If an ADU occupies an entire single floor, the planning director may allow for an increase in the size of the ADU in order to efficiently use all the floor area. An ADU is permitted above detached garages, or attached garages, if the ADU area is not in
excess of 1,500 square feet. ADUs may not be subdivided or otherwise segregated in ownership from the primary dwelling unit. One off-street parking space must be provided. Owner occupancy is required in one of the two units.
library.municode.com/wa/duvall/codes/code_of_ordinances?nodeId=CO_TIT14UNDERE_CH14.48ACDWUNAD_14.48.030RE
EDMONDS
One AADU allowed. DADUs are not allowed. “Attached” is defined as a building located within 10 feet of the main building, as measured from exterior wall to exterior wall, and connected by a breezeway, hallway, or similar connection. The AADU must not exceed 40% of the livable floor area of the principal dwelling. The ADU must not be more than 800 square feet or have more than two bedrooms. Only one electric and one water meter shall be allowed for the entire building, serving both the primary residence and the ADU. Requires additional off-street parking spot, for a total of three spots. Owner occupancy is required in one of the two units.
edmondswa.gov/images/COE/Services/Permits_and_Development/Forms_and_Handouts/P1--- AccessoryDwelling-Unit.pdf
EVERETT
One AADU or DADU allowed. The ADU may be for a single-family home, duplex, triplex, or townhome. If the primary dwelling unit is a single-family home, the ADU size must be the lesser of 1,000 square feet, 15% of the total lot area or the principal dwelling’s building footprint. If the primary dwelling unit is a
duplex, triplex or townhome, the ADU size must be the lesser of 7.5% of the lot area, 37.5% of the gross floor area or 440 square feet. Owner occupancy is required in either the primary residence or the ADU.
everettwa.gov/DocumentCenter/View/26221/Ch-8_Residential_Summary • (pages 4-5)
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 is prohibited from having 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. No more than one single “housekeeping” unit may occupy an ADU.
codepublishing.com/WA/FederalWay/#!/html/FederalWay19/FederalWay19200.html • (19.200.180)
GRANITE FALLS* • CURRENTLY UNDER BUILDING MORATORIUM
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. codepublishing.com/WA/Issaquah/#!/Issaquah18/Issaquah1807.html#18.07.450 • (18.07.450)
KENMORE*
One AADU or DADU allowed. DADUs on lots >6,000 square feet are limited to 10%, up to 1,500 square feet. DADUs on lots <6,000 square feet are limited to 600 square feet. AADUs are limited to 1,000 square feet or a single floor. There is a six-month owner occupancy requirement. The maximum height is limited to 35 feet or two stories.
kenmore.civicweb.net/FileStorage/0BE1729A8C624570BB149870016B1FB5-Attachment%201%20Part%202%20-%20Revised%20Ordinance%20Attachment.pdf
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.
kentwa.gov/Home/ShowDocument?id=4092
KIRKLAND*
Two ADUs allowed. One AADU and one DADU, or two AADUs/DADUs per single-family dwelling unit. Two DADUs are prohibited in certain cases. Duplexes can have ADUs. ADUs are limited to 1,200 square feet. If the AADU is completely located within existing gross floor area on a single floor, the Planning Director may allow increased size in order to efficiently use all floor area. On lots with more than one
ADU, there shall be one off-street parking space provided, unless a) on-street parking is available within 600 feet of the subject property or b) the property is located within one-half mile of transit service with 15-minute headways during commute hours. No owner occupancy requirement.
kirklandwa.gov/Government/Departments/Planning-and-Building/Housing/Accessory-Dwelling-Units
LAKE FOREST PARK
One AADU or DADU allowed. ADUs are only allowed on lots of at least 7,200 square feet and only AADUs are allowed on lots of less than 15,000 square feet. The ADU floor area must be at least 300 square feet but may not exceed 50% of the total area of the primary dwelling unit, or 1,000 square feet, whichever is less. One off-street parking space required. Owner occupancy is required in one of the two units.
cityoflfp.com/DocumentCenter/View/6301/25-Accessory-Buildings_ADU
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 requirement. Owner occupancy is required in one of the two units.
lakestevenswa.gov/DocumentCenter/View/6661/1030-Accessory-Dwelling-Units?bidId=
LYNNWOOD
One AADU allowed in properties zoned R-7. One AADU or DADU allowed in properties zoned R-8. 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.
lynnwoodwa.gov/files/sharedassets/public/development-and-business-services/planning-amp- zoning/applicationsand-checklists/accessory-dwelling-units/2.-adu-guide_2018_update-draft.pdf
MAPLE VALLEY
One AADU or DADU allowed. Only AADUs are allowed if the lot is less than 10,000 square feet. The ADU shall not be larger than 50% of the living area of the primary residence. One off-street parking space required. Owner occupancy is required in one of the two units.
maplevalleywa.gov/home/showdocument?id=18237
MARYSVILLE
One AADU or DADU allowed. The floor area of the accessory dwelling unit shall not exceed 35% of the total floor area of the single-family dwelling. Additional off-street parking spot required. Owner occupancy required. If the owner ceases to reside in either the principal or the accessory dwelling unit due to illness,
death, or some other unforeseeable event preventing continued occupancy, the community development director may, upon finding that discontinuance of the accessory dwelling unit would cause a hardship for the owner and/or tenants, grant a temporary suspension of this owner occupancy requirement for a
period of one year.
codepublishing.com/WA/Marysville/#!/Marysville22C/Marysville22C180.html
MEDINA
One AADU or DADU allowed. DADUs must be located within a detached accessory building containing another permitted accessory use—DADUs are prohibited as the only use in a detached accessory building. The ADU must be no more than the lesser of 1,000 square feet of gross floor area or 40% of the total square footage of the gross floor area of the single-family dwelling and ADU combined. The
entry door to the ADU shall be screened from the street by portions of the structure or by dense evergreen vegetation. One additional parking space and owner occupancy are required.
codepublishing.com/WA/Medina/#!/html/Medina20/Medina2034.html• 20.34.020
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. Accessory dwelling units shall not be subdivided or otherwise segregated
in ownership from the primary dwelling unit. Primary dwelling unit or ADU must be occupied by an owner of the property or an immediate family member of the property owner.
mercerisland.municipal.codes/MICC/19.02.030
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.
codepublishing.com/WA/MillCreek/html/MillCreek17/MillCreek1722.html
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. See also: Accessory Dwelling Units—Brochure #40 by City of Mukilteo.
codepublishing.com/WA/Mukilteo/html/Mukilteo17/Mukilteo1730.html
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 cannot 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. The ADU may not be subdivided or otherwise segregated in ownership from the primary dwelling unit. Owner occupancy is required in one of the two units.
codepublishing.com/WA/Newcastle/html/Newcastle18/Newcastle1831.html
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. The height may not exceed 25 feet. ADUs cannot be separated or subdivided in ownership from the primary dwelling unit. Owner occupancy is required in the primary dwelling unit.
codepublishing.com/WA/NorthBend/html/NorthBend18/NorthBend1810.html#18.10.041
PACIFIC
One AADU or DADU allowed. The AADU must contain no more than 45% of the floor area of the primary dwelling unit. (Garage and staircase areas shall not be included in the calculations.) DADUs may be no more than 800 square feet within the RS-6 (single-family residential, 6,000 square feet) zoning district and 1,000 square feet if located within the RS-11 (single-family residential, 11,000 square
feet) zoning district. Two off-street parking spaces are required if the ADU has two bedrooms and one off-street parking space if the ADU has one bedroom or it is a studio ADU. Owner occupancy is required in one of the two units.
codepublishing.com/WA/Pacific/html/Pacific20/Pacific2092.html
REDMOND
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.
redmond.gov/DocumentCenter/View/7503/Accessory-Dwelling-Unit-PDF
RENTON*
One DADU allowed. No AADUs allowed. The size of allowed DADU is based on the lot area, and is determined as follows:
Lot Area Maximum DADU Size
3,000 square feet or less 600 square feet
3,001-4,999 square feet 700 square feet
5,000-6,999 square feet 800 square feet
7,000-8,999 square feet 900 square feet
Greater > 9,000 square feet 1,000 square feet
In most cases, ADUs will require a minimum of one additional parking space and a maximum of two is allowed. Owner occupancy is required unless the applicant applies for and is granted a Conditional Use Permit (CUP).
edocs.rentonwa.gov/Documents/1/edoc/967512/Accessory%20Dwelling%20Units.pdf
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.
sammamish.us/attachments/pagecontent/36778/17280.pdf
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. ADUs created within the single-family structure shall not be subdivided or otherwise segregated in ownership from the primary dwelling unit. Detached ADUs may be segregated in ownership from the
primary dwelling unit if such segregation meets all minimum requirements for a separate legal lot under City of SeaTac zoning and subdivision standards. For a minimum of nine months in any 12-month period, the owner of the primary unit must occupy one of the two units.
codepublishing.com/WA/SeaTac/#!/html/SeaTac15/SeaTac15465.html • 15.465.100
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: 1) green building standard or 2) be 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.
seattle.gov/sdci/permits/common-projects/accessory-dwelling-units
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. Fire sprinklers are required in a DADU. The owner of the property or an immediate family member must live in either the main house or the ADU.
shorelinewa.gov/home/showdocument?id=2364
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.
snohomishwa.gov/DocumentCenter/View/208/ADU-FAQ
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.
codepublishing.com/WA/Snoqualmie/html/Snoqualmie17/Snoqualmie1715.html
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.
codepublishing.com/WA/Stanwood • (17.95.470)
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. Height limit is 20 feet except for apartments built over a garage, which may be up to 25 feet tall. One additional parking space is required. Owner occupancy is
required in one of the two units.
tukwilawa.gov/wp-content/uploads/DCD-Planning-Accessory-Dwelling-Units.pdf
UNINCORPORATED KING COUNTY
One AADU or DADU allowed in a single detached dwelling or townhome unit.
DADUs are allowed when:
• There is no more than one primary dwelling unit on the lot
• The lot is 3,200 square feet or greater if located in the urban area or a rural town; OR
• The lot must meet the minimum lot area for the applicable zone if located in the rural area but not in a rural town, except that if one transferable development right is purchased from the Rural Area or Natural Resource Lands under K.C.C. chapter 21A.37, a DADU unit is allowed on a RA-5 zoned lot that is two and one-half acres or greater.
The ADU can’t be more than 1,000 square feet of heated floor area and 1,000 square feet of unheated floor
area except:
• When the ADU is wholly contained within a basement or attic
• For DADUs, the floor area contained in a basement does not count toward the floor area maximum; OR
• On a site zoned RA if one transferable development right is purchased from the Rural Area or Natural Resource Lands under K.C.C. chapter 21A.37, the ADU is permitted a maximum heated floor area of 1,500 square feet and 1,500 square feet of unheated floor area.
When the primary and accessory dwelling units are in the same building, or in multiple buildings connected by a breezeway or other structure, only one entrance may front a street. No additional parking spaces required. The primary dwelling unit or the ADU must be occupied either by the owner of the primary
dwelling unit or by an immediate family member of the owner. Immediate family members are limited to spouses, siblings, parents, grandparents, children, and grandchildren, either by blood, adoption, or marriage, of the owner.
kingcounty.gov/~/media/depts/permitting-environmental-review/dper/documents/forms/ResidentialAccessory-Dwelling-Units-Information-old.ashx?la=en
kingcounty.gov/council/legislation/kc_code/24_30_Title_21A.aspx • (21A.08.030)
UNINCORPORATED SNOHOMISH COUNTY
Inside Urban Growth Areas
Two ADUs allowed: one AADU and one DADU. ADUs can be up to 1,200 square feet. No parking requirement or owner occupancy requirement.
Outside Urban Growth Areas (Rural)
One AADU or DADU allowed. DADUs allowed only on lots that meet the minimum lot size.* ADUs can be up to 1,200 square feet. A maximum distance between the primary dwelling unit and a DADU is 100 feet. ADUs must utilize the same driveway as the primary dwelling unit. One additional parking space is required. No owner occupancy requirement. *Currently there is unfinished business related to DADUs on certain rural lots. The matter was referred to the Council’s Planning Committee for further review. Currently there is still a prohibition on DADUs on lots
in rural areas that do not meet the minimum required lot area for the zone in which they are located (see SCC 30.28.010).
snohomishcountywa.gov/DocumentCenter/View/80953/Hearing-Packet
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.
codepublishing.com/WA/Woodinville/#!/Woodinville21/Woodinville2121.html • (21.21.030)
