Who is this guide for?

Hello there, and welcome to this info page about all things related to backyard cottages and in-law apartments in Seattle. We put this guide together for you, the Seattle homeowner interested in learning more about building a cottage in your backyard or an apartment in your basement. Perhaps you want some rental income or just have some extra space. If so, a backyard cottage or ADU can be a great choice. The regulations can be a bit confusing, but Seattle has recently implemented updates that make it easier to add additional housing. There's probably more information here than you can take in, but we hope you can use it to get ideas and get started on your backyard cottage project. If there's anything you think should be included in this guide, we'd love to hear about it. You can email hello@crddesignbuild.com. Be sure to check back frequently or sign up for updates because we'll be adding new information as it becomes available.

Section 1

Getting Started

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Illustrations: City of Seattle

What is an in-law apartment?

Sometimes called “mother-in-law apartments,” accessory dwelling units, or ADUs, these are self-contained living spaces built into your home. To create these apartments, you can convert your basement by adding an outside entrance, egress windows, and a small kitchen, or you can add on to the back or side of your home. To be official, an ADU must have its own private entrance and meet certain other requirements.

Screen Shot 2017-11-22 at 11.19.10 AMWhat is a backyard cottage?

Detached accessory dwelling units, or DADUs, are small, stand-alone houses built behind your main home. They allow you to maintain your privacy and all the square footage in your main house while creating extra living space. DADUs are also known as "backyard cottages." Homeowners often opt to build these over a garage.

What are the benefits of backyard cottages?

There are many benefits to building these units alongside an existing single-family home or townhome.

  • Backyard cottages provide many of the benefits of single-family homes, with no shared walls and a lower-density-neighborhood lifestyle.
  • They can promote economic diversity in neighborhoods that might be out of range for average-income renters.
  • ADUs and DADUs can provide passive rental income for the homeowner. This can help homeowners of modest means stay in their homes or simply put extra cash in your pocket to help pay the mortgage or pay for home improvements.
  • DADUs are an example of infill development, which slowly increases density and taking advantage of existing infrastructure. Both ADUs and DADUs can help reduce sprawl.
  • Seattle's population is growing due to economic opportunities here, and we must increase our housing stock to keep pace. When compared to highrise developments, ADUs and DADUs are a solution that helps preserve the character of our beloved neighborhoods while empowering homeowners instead of large developers.

Helpful Links

Section 2

Seattle Regulations

So you want to build a backyard cottage or ADU? Great! The first step is to make sure you know all the rules. Seattle legislation adopted new rules for ADUs and DADUs in 2019 that were designed to make the process of adding an ADU or DADU smoother and more equitable for Seattle homeowners. Here's a breakdown of some of the latest regulations:

ADUs (In-Law Apartments)

Attached accessory dwelling units (ADUs) or in-law apartments have been allowed in all single-family homes in Seattle since 1994.  As of 2021, there were 2,582 permitted ADUs, and recent changes in Seattle legislation are helping this number continue to grow, as it becomes easier to gain approval on new construction. Here's a rundown of the current ADU regulations:

  • Size limit: ADUs must be 1,000 square feet or under in a single-family home or 650 square feet and under in a rowhouse or townhouse. Good news - this square footage excludes the garage area.
  • Floor area: More good news, all ADUs are exempt from floor area ratio (FAR) calculations.  
  • Meet codes: The ADU must meet current standards of the Seattle residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shorelines codes.
  • Double up: Homeowners can have up to two ADUs in SF zones.
  • Parking: Previously, homeowners were required to provide one off-street parking space for the ADU, but as of 2019, this is regulation is no longer required. This change offers some cost savings to homeowners since even a simple gravel parking space could cost homeowners thousands of dollars to build. 

DADUs (Backyard Cottages)

Examples of site configurations for backyard cottagesSeattle City Council

Backyard cottage regulations are a bit more complicated than those for in-law apartments. The city started a pilot program in 2006, allowing DADUs to be built in Southeast Seattle. It was considered a success, and the city expanded the program to include all of Seattle in 2009. As of 2021, 1,346 backyard cottages have been permitted of the roughly 75,000 single-family lots that are eligible, with a large spike in the number of permits in 2020, despite the pandemic. With the new 2019 regulations in place, it appears the demand for DADU permits will continue to grow.

DADU permits as of 2021

City of Seattle - ADUniverse Data Reporting 


Current Backyard Cottage Requirements

If you're thinking about investing in a backyard cottage, you may want to check with a knowledgeable builder or architect about your eligibility or refer directly to  Seattle Municipal Code  23.44.041. Here is a summary of the current regulations:

  • Lot size: Your lot must be at least 3,200 square feet in a single-family zone. Your lot must be at least 25 feet wide and 70 feet deep
  • Lot coverage: If your lot is under 5,000 square feet, the structures on your property (your primary residence combined with a DADU) can cover 1,000 square feet plus 15% of your lot area. If your lot is larger than 5,000 square feet, your primary residence and DADU are allowed to cover up to 35% of your lot area. Certain parts of structures are exempt from the calculation, including some decks and eaves projecting from your house. 
DADU lot coverage allowance varies by the width of your property. Check the municipal code here for lot size specifics. 
  • Setbacks: DADUs should be built in the rear yard, and are not allowed in the front yard.  You'll need five feet of space between the DADU and your lot lines and other structures. If you have an alley behind your lot, you can build right up to it. Some exceptions to these rules apply in various SF zones, so we recommend checking your specific address with the Property Search tool on the City of Seattle's resource page before you begin your project. 
  • Rear yard coverage: Your backyard cottage can cover up to 60% of the rear yard (up from 40%) depending on your tree coverage. See section 23.44.014.D1 for tree-removal details.
  • Cottage size: 1,000 square feet is allowed for DADUs in single-family zones and 650 square feet for units in a lowrise zone. If your DADU incorporates a garage or storage area, that square footage no longer counts toward your total, thanks to the updated 2019 regulations.
  • Height limit: The maximum height limit of your DADU will depend on the width of your lot and is divided between the height of your base structure and your roof type. Various roof types are allowed different limits. For example, if the width of your lot is between 30 and 40 feet, the maximum height of your base structure can be 16 feet. If you choose a pitched roof, that portion of your DADU can not exceed seven feet, bringing your total height allowance to 23 feet. See our chart below for more height combinations:
    DADU Height Limit Chart2
  • Codes: As with any construction project, your backyard cottage must meet current land use and building standards. No surprise here.
  • Parking: Like the updated regulations for in-law units, backyard cottages are no longer are required to have an off-street parking space.

Helpful Links


Section 3

Regional Regulations

If your home is located outside of Seattle city limits, you may still be able to build an in-law apartment or backyard cottage. The information about Pacific NW ADU and DADU regulations was compiled by the Master Builders Association of King and Snohomish Counties. It is important to verify this information with your local building authority. Download a PDF summary of regional accessory dwelling regulations. Follow their Twitter hashtag #COTTAGEPALOOZA for updates.


40 percent of the total square footage of the residence for ADUs. Not less than 300 square feet or greater than 800 square feet. Owner occupancy required in one of the two units. One off-street parking space required. ADUs not currently allowed in new construction.
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If detached: 800 square feet or 33 percent of the size of the principal unit, whichever is smaller. If attached: not more than 40 percent of the principal unit.
One off-street parking space. One of the dwelling units must be owner-occupied for at least six months a year. Interestingly, Kent does allow ADUs in both new and existing single-family dwellings.
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75 percent of gross area of the principal unit or 800 square feet, whichever is less. Most have design standards requiring consistency with the primary residence. Owner must reside within either dwelling. Requires additional off-street parking stall unless within a quarter mile of a transit stop.


No larger than 75 percent of the primary residence or 800 square feet, whichever is smaller. Owner must occupy one of the two units. 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 percent and waived in full for every third new ADU. This incentive was extended through 2020.
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One accessory unit allowed per single-family dwelling. May be within or detached from the principal dwelling unit. The primary unit or the accessory unit must be owner occupied. An attached accessory unit cannot exceed 40 percent of the total area of the principal residence and the ADU. The size of a detached ADU may not exceed 800 square feet. The total area of all detached accessory structures on your property may not exceed 1,200 square feet plus 10 percent 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 required, in addition to the two required for the primary residence. Primary entrance to the ADU must be clearly secondary to the main entrance of the principal unit.
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ADUs are not to exceed 50 percent of the primary residence or 950 square feet, whichever is less. They must match primary residence. One additional parking space must be provided and owner occupancy is required in one of the two units.


15 percent of total lot in R4.5, R6.5, R8, WR R4-8; 20 percent of total lot in R12-28 and WR6-18 zones; or 80 percent of primary floor area, whichever is less. Legal owner must reside within either dwelling. Requires additional off-street parking stall.


Size is limited to 50 percent of the size of the main dwelling. Owner occupancy required in one of the two units. One off-street parking space in addition to two for the main house.
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Size must be 50 percent of gross primary floor area or 800 square feet max, whichever is less. Legal owner must reside within either dwelling. One additional off-street parking space, in addition to existing SFR requirement.


City updated outdated ADU regulations this year and removed construction barriers to build with minimal impact in existing residential neighborhoods.


On December 2, 2019, the Burien City Council passed reforms to loosen the restrictions to build ADUs in single-family zones. Read the ordinance here.

See also: Burien Encourages Accessory Dwelling Units in New Reform [The Urbanist]


Adopted regulations earlier this year to allow ADUs or DADUs up to 800 square feet in order to increase housing stock.


Has a Residential 2 (R2) zone that allows for the construction of duplex, triplex, quadruplex, attached townhomes of no more than 4 units, and small ADUs and DADUs.


Size to be 50 percent of gross primary floor area or 800 square feet max, whichever is less. Must comply with city design standards. One unit occupied by the land owner. One additional parking space required, for a total of three parking spaces.


The city is in the process of reviewing, amending, and updating its ADU regulations with a targeted completion date and anticipated vote at council in January 2019.


Size to be 60 percent of primary residence or 900 square feet, whichever is less. Design elements must be consistent with main house. Legal owner must reside within either dwelling. One additional parking space must be provided.


Updated their cottage residential regulations after a moratorium on their construction. Cottages must be under 1,500 square feet, clustered in development, and their design must promote a sense of openness and community. All homes must have easy access to a central common area or open space and, where
possible, be designed with a covered porch and primary area facing the common area to encourage interaction and community.


In March 2019, Tacoma legalized backyard cottages and relaxed restrictions on in-law apartments (ADUs). They are exempted from off-street parking requirements. The owner is not required to live on the property, unless the ADU is used for short-term rentals. No minimum lot size. Allows for zero setback for DADUs on alleys.
Learn more

See also: Tacoma Adopts Exemplary Reform for In-Law Apartments [Sightline Institute]

Section 4

News & Inspiration







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Get ADU/DADU Updates

Stay up to date on the latest in-law apartment and backyard cottage news. Want to explore CRD's full-service design-build approach to backyard cottages and ADUs, please contact us.