On Tuesday, the City of Seattle rolled out a website called ADUniverse that will offer 10 pre-approved accessory dwelling unit (ADU) designs and other information to help homeowners navigate the process of adding a backyard cottage or mother-in-law apartment (or two) to their lot.

“In addition to providing an easily accessible design option, the pre-approved designs will shorten the permitting process by at least 2-6 weeks and save homeowners about $1,500 in permit fees,” the Mayor’s office said in a press release. Nick Welch, senior planner with the Office of Planning and Community Development (OPCD), said that time and monetary savings could be even greater in some cases by avoiding costly corrections to initial designs.

Last summer, the Seattle City Council passed ADU reform–but only after a lengthy delay caused by a Queen Anne Community Council appeal forcing the City do a full Environmental Impact Statement (EIS) and then appealing the EIS by ironically arguing under the State Environmental Policy Act (SEPA) that more parking was needed, a case the Hearing Examiner ultimately rejected. The new law allows two ADUs on large lots and makes ADUs feasible on more lots than previous rules. Sightline Institute declared Seattle’s new ADU policy, which former Councilmember Mike O’Brien had shepherded, the best-in-the-nation.

After signing the law, Mayor Jenny Durkan directed City agencies to identify strategies to reduce permitting times and costs. The City launched an effort to get pre-approved ADU designs, appealing to the architect community and originally floating compensation for those designs as a possibility. Ultimately, the City did not offer compensation when they put out the call in December, deciding it was enough that firms get their name out there and get the $1,000 in royalties each time their design is built.

Architect Mike Eliason opined that the City’s pre-approved plan effort was built on uncompensated labor in a column in The Urbanist. Welch said the City got an overwhelming response with 165 designs submitted–even without upfront compensation for architects. All the designs will appear on the site to recognize their participation in the process and perhaps drum up business for those not so lucky to win.

“All architectural plans–ranging from a studio under 300 square feet to a 1,000-square-foot two-bedroom–will be available direct from the designer for $1,000 or less,” the Mayor’s release said. “Cottage designs have been reviewed against codes for the structure and its energy use; however, homeowners remain responsible for permits and inspections related to zoning, site preparation and the foundation, utility connections, and other site-specific requirements.”

Rendering and floorplans for the FiveDot ADU
FiveDot’s backyard cottage was a 1,000 square foot two bedroom, the largest pre-approved design.

In other words, though the design will be pre-approved and $1,000 or less, a considerable amount of fees and hoops to jump through remain, including a site plan. Eliason pointed out the architectural design costs associated with an ADU are typically on the magnitude of $15,000 all told. Welch said the firms with a pre-approved design would recoup the $1,000 in royalties when their designs are built, plus they may stand to get the business from the homeowner to get the site plan and everything else in order, adding more compensation for their work.

The pre-approved plans were selected with affordability and sustainability as major factors of consideration. A panel of volunteers from the Seattle Design Commission, Seattle Planning Commission, Construction Codes Advisory Board, and the City’s design review boards identified 10 designs for permitting by the Seattle Department of Construction and Inspections (SDCI). “The designs were selected based on estimated cost of construction, accessibility, green building principles, and compatibility with historical architecture,” the Mayor’s release notes. The City had hoped to release the pre-approved designs sooner but the coronavirus pandemic threw a wrench in that plan, officials say.

The winning firms were CAST Architecture, Magellan Architects, Fivedot Architects, Artisan Group, Ahouse Studio, Urban Cottage Prefab and Wood Studio, Yes Architecture, Mobile Office Architects, Bohlin Cywinski Jackson, and Shape Architecture.

A flat-roof cottage with cedar tree in front yard.
CAST Architecture’s Cedar Cottage is 467 square feet and promises relatively low cost.

“Our technical staff have completed reviews on a range of cottage designs and sizes, which will reduce the time it takes to get started on construction,” SDCI Director Nathan Torgelson said in a statement.  “Anyone thinking of adding a backyard cottage now has access to better information and an immediate head start on obtaining a permit.” 

A pre-approved design can’t do anything about utility fees, however, which do add a significant cost. One large fee is the sewer capacity fee levied by King County, but Welch said the City worked with the County to make it a little bit lower so that ADUs aren’t paying as much as full-sized single-family homes. That change took effect about a year ago. The sewer hookup fee had been $7,000 paid in installments over 15 years, but the City convinced the County to lower that a bit for ADUs.

What the ultimate effect pre-approved ADU designs will have on the affordability of housing may be fairly incremental. It’s more of a middle-income housing solution, but affordable housing providers may be able to use ADUs here and there. The Mayor is optimistic it can make a dent.

Graph of ADU production next to a map showing locations
The ADUniverse website includes a data dashboard. Attached ADUs shown in light blue, and detached ADUs in dark blue. ADU production peaked in 2016 with 274, but the City’s ADU reform aims to push that number higher. (City of Seattle)

“While in the middle of global health pandemic, we are even more aware that we continue to face an affordability and housing crisis. It’s our responsibility to use every tool available to increase housing options in every part of our City. While the City expects to build more than 6,600 new affordable homes by 2023, we need to ensure we have range of housing options for everyone,” Mayor Durkan said. “These new designs will help streamline permitting issues, allow homeowners to provide alternative housing choices to renters within their communities, and create options for residents to age in place or live with loved ones.” 

Check out the new ADUniverse website if you’d like to see for yourself. It includes a parcel viewer that can help determine if a backyard cottage can fit on your lot. There is also a “Data” section that shows historic ADU permit activity citywide. Activity hasn’t spiked much yet from ADU reform passage, but it may soon as homeowners get their plans together.

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Doug Trumm is The Urbanist's Executive Director. An Urbanist writer since 2015, he dreams of pedestrianizing streets, blanketing the city in bus lanes, and unleashing a mass timber building spree to end the affordable housing shortage and avert our coming climate catastrophe. He graduated from the Evans School of Public Policy and Governance at the University of Washington. He lives in East Fremont and loves to explore the city on his bike.

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That 1K sq ft ADU is huge; it’s bigger than a traditional craftsman. What’s the max size for a DADU or ADU? At that size, whats the difference between a DADU and simply subdividing the lot into a separate home?

Douglas Trumm

1,000 square feet is the new cap for both attached and detached ADUs. While maybe traditionally mid-century Craftsmen were small, today the average size of a new single family home is about 2,600sf. From I can tell, new Seattle Craftsmen aren’t an exception to that. Even at 1,000sf, ADUs are still much smaller than most single family homes. https://www.theurbanist.org/2019/07/03/what-seattles-best-in-the-nation-mother-in-law-apartment-and-backyard-cottage-reform-does/

As for subdividing the lot, from I understand that’s a complicated/expensive process. Adding a DADU is probably simpler from a homeowner perspective.


Yeah by ‘traditional’ I meant the old, 1-story early & mid-century ones scattered around Seattle.

Got it, so the big difference is it remains a single lot with a single owner?


“whats the difference between a DADU and simply subdividing the lot into a separate home?”

Purely politics. We should allow really small subdivisions, which in turn would allow a lot more people to buy (or rent) their own (small lot) house.


Totally agree. It is possible now to condo-ize a DADU and the main house, allowing for separate ownership.

In our neighborhood, a 1,000 square foot house on a legacy 2,500 square foot lot sold last year for $775k. That leaves a lot of daylight for a 1,000 square foot DADU built for 300 / sf ($300,00) to sell for much less – whether at cost to family or as a commercial venture.