Illustrations: City of Seattle
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, but the regulations can be a bit confusing, and Seattle is in the midst of changing them. 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 email@example.com. Be sure to check back frequently or sign up for updates because we'll be adding new information as it becomes available.
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.
Detached accessory dwelling units, or DADUs, are small, standalone 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.
There are many benefits to building these units alongside an existing single-family home or townhome.
So you want to build a backyard cottage or ADU? Great! The first step is to make sure you know all the rules. There are some major changes to the regulations in the works, but let's take a look at the current regs.
Attached ADUs (in-law apartments) have been allowed in all single-family homes in Seattle since 1994. As of 2016, there are just over 1,000 units. Here is a rundown of the current ADU regulations:
Backyard cottage regulations are more hotly contested and 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. However, as of 2016, only 221 backyard cottages have been built out of the roughly 75,000 single-family lots that are eligible.
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 23.44.016.D.8. Here is a summary of the current regulations:
Seattle City Council
Back in May of 2016, Councilmember Mike O'Brien proposed new regulations that would make it easier to build backyard cottages and in-law units in Seattle. Here is a rundown of the proposed changes:
Seattle City Council
The Office of Planning and Community Development issued a State Environmental Policy Act (SEPA) Determination of Non-Significance (DNS) in May of 2016 on my proposal. Following the release, the Queen Anne Community Council appealed the DNS to the hearing examiner. This past December we received the response from the Hearing Examiner that reversed the DNS. After thorough examination of the Hearing Examiner’s response, we have decided to pursue a full environmental impact statement (EIS). This process will likely take a year to complete. The full EIS will enable us to look deeply into the potential environmental impacts of the proposed code changes and inform our proposal before we bring it to the Planning, Land Use and Zoning Committee.
There will be multiple opportunities for input during the EIS process. We will keep you informed of these opportunities for public comment and encourage you to engage. When the EIS is complete, we hope to bring legislation to committee by mid-2018.
I believe lowering the barriers to creating backyard cottages and in-law apartments is an important part of addressing affordability across the city, and am looking forward to continuing to pursue this legislation.
If you have further questions, please reach out to Susie Levy – firstname.lastname@example.org or call our office at 206-684-8800.
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.
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.
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.
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
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.
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
City updated outdated ADU regulations this year and removed construction barriers to build with minimal
impact in existing residential neighborhoods.
Adopted regulations earlier this year to allow ADUs or DADUs up to 800 square feet in order to increase
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.
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.