Seattle City Council took a big step Monday toward creating a more sustainable city, voting unanimously to enact legislation that will make it easier for homeowners to build in-law suites, garage apartments, and backyard cottages—modest homes the wonks call accessory dwelling units (ADUs). 

The vote caps an epic process during which obstructionists abused state environmental laws to drag things out for four years, as pro-housing affordability forces built up steam and finally won out over the objections of a tiny minority of anti-housing activists. 

The adopted changes amount to the most progressive ADU policy in the US. By boosting the addition of ADU homes in the three-quarters of Seattle’s residential land reserved for expensive, single-detached houses, the new rules will lower the invisible walls of economic exclusion, and make the city greener too. 

Two ingredients set Seattle’s policy apart: an allowance for not just one but two ADUs per lot, and a size limit on new houses. The only other major North American city permits two ADUs in single-dwelling zones is Vancouver, BC.* This flexibility maximizes opportunities for smaller, less expensive homes in neighborhoods otherwise filled with big, stand-alone houses. 

Last year Minneapolis made national headlines with its plan to triplex the city. Depending on how Minneapolis implements that plan, Seattle’s new allowance could yield just as big an advance for welcoming moderate-income families to desirable communities.

The legislation’s house size limit of 2,500 square feet on a typical lot will make it more attractive to add ADUs to existing houses instead of tearing those houses down. And when teardowns inevitably occur—as they already do under the status quo—the size cap will provide a new incentive to build ADUs along with the new house. Neighborhoods will see fewer teardowns replaced by McMansions and more modest, mixed-income options. 

Seattle also got rid of the two biggest ADU barriers: parking quotas and the requirement for property owners to live on site. For cottages, the legislation bumps up allowed size, height, and backyard coverage, making it easier to build family-friendly homes and lowers the minimum lot size to 3,200 square feet. And to avoid discriminating against non-traditional family arrangements, it raises the limit on unrelated occupants per lot from eight to twelve. 

The council rejected an amendment that would have hindered ADU production by requiring a year of property ownership before granting permits. In a previous press conference, Councilmember Mike O’Brien took issue with the unspoken sentiment embedded in such rules: “This idea that somehow, you know, homeowners are better citizens on our streets than renters are, is just something I disagree with.” 

Council also refrained from enacting a proposed outright ban on using ADUs for short-term rentals. Seattle already caps all short-term rentals at two per owner. Fully banning short term rentals in ADUs would quash flexibility and dissuade homeowners from building them. 

Add it all up, and Seattle’s ADU policy is as good as it gets—Vancouver, BC, is the only other large North American city with a comparably liberal set of rules. Cities throughout Cascadia and beyond now have a living example of a way to craft policy that embraces ADUs and maximizes their benefits to both people and the planet.

Adu backyard cottages

Backyard cottage in Seattle’s Matthews Beach neighborhood. Photo by CAST Architecture, used with permission.

Anti-housing obstructionists turned common-sense ADU reform into a marathon

Sightline has long supported ADU reform, starting with research in 2013 that called out regulatory barriers. Sightline’s executive director served on Seattle’s Housing Affordability and Livability Agenda committee and helped ensure that the 2015 report included the needed changes to Seattle’s rules—recommendations (see page 25) that match the key fixes in the ordinance the city just adopted.   

Councilmember Mike O’Brien proposed ADU legislation in late 2015 (here’s a timeline). Progress was stalled for a total of three years by two legal appeals filed by the Community Council of Queen Anne, a leafy Seattle neighborhood where the median home price is just shy of $1 million. 

The delay did have one upside, though: it gave policymakers time to improve the original proposal by axing all ownership requirements, adding the option for two ADUs attached to the main house, and introducing a brand new approach: a size limit on new houses to tip the redevelopment scales away from McMansions and toward more ADUs. It also gave planners the opportunity to conduct extensive analysis that debunks specious claims commonly made by ADU detractors. 

In May, nearly four years after city officials first called for ADU reform, the last appeal was dismissed, clearing the way for today’s unanimous vote led by Councilmember O’Brien. Over those four years, Sightline published a plethora of supporting research (scroll to the bottom of the article for a reading list).

Seattle wins with greener, more affordable housing choices

The new rules will yield more of the very type of homes Seattle desperately needs as it stares down both a climate and an affordability crisis. Nearly half of the city’s dwellings are stand-alone houses on large lots that are not only pricey, but also energy guzzlers. Low-density neighborhoods filled with block after block of big houses are a set up for heavy car reliance

Because ADUs are compact, on average they use 40 percent less energy than a typically-sized, single-detached house. Over a home’s lifespan, that’s equivalent to taking nearly 100 cars off the road for a year. Tucking ADUs into existing neighborhoods also supports more walkable, transit-friendly communities, cutting climate pollution. 

  • Our work is made possible by the generosity of people like you!

    Thanks to Richard Ackley for supporting a sustainable Cascadia.

  • In addition to being economical because they are small, ADUs also tend to rent below market-rate, and 1 in 10 provides free housing to family or friends of the homeowner. Seattle’s reform is well-timed for the city’s growing population of seniors, many of whom hope to age in place. ADUs offer a practical option for adding age-friendly housing to established neighborhoods to help seniors stay in the communities they love.  

    backyard cottages ADU housing affordability

    Backyard cottage tucked behind single-story house.

    How Seattle could polish up its already exemplary ADU policy

    Notwithstanding Seattle’s stellar achievement on ADU reform, as think-tankers we’d be remiss not to suggest a few small ways in which policymakers could make it even better:

    • Remove the requirement that the second ADU must either meet a green building standard, or be reserved for income-eligible households: These extra mandates will make it more financially challenging to construct ADUs—counterproductive policy for a housing type that is relatively green and affordable to begin with.
    • Reduce or eliminate permitting and utility connection fees: These added costs can be deal breakers for homeowners on tight budgets (the most onerous fee on ADUs in Seattle is King County’s sewer connection charge). 
    • Allow the option of two backyard cottages, attached to each other, or detached: The same size limits that apply to a single cottage would apply to two cumulatively, so the built structure couldn’t be any bigger.
    • Allow ADUs inside the main house to exceed 1,000 square feet: Exempt the first 1,000 square feet of ADU from the total house size limit, but allow the owner to divide the area inside the house however they like. Vancouver, BC, caps attached ADU size at no larger than the main house.
    • Eliminate the minimum lot size requirement: Lot coverage limits and setback requirements preclude cottages on most smaller lots anyway, but some small lots—ones with very small houses, for example—could easily accommodate a backyard cottage.
    • Remove occupancy limits based on legal relationship status: Over recent decades family structures have evolved and tend to be less conventional. Barring additional residents because they are not legally related is discriminatory.

    The wave is swelling to end exclusionary zoning

    Today’s progressive move to relax restrictions on ADUs is a big first step in a necessary evolution toward opening up Seattle’s expansive low-density residential areas to less expensive homes, and more of them. Momentum is spreading: earlier this year, Washington state legislators proposed a bill (destined to fail) that would have mandated rules similar to Seattle’s in larger cities statewide. California’s legislature is currently considering a bill that would enable cities to permit more than one ADU per lot.  

    To build on its success with ADU policy, Seattle can next take on re-legalizing middle housing—triplexes, townhouses, and small apartment buildings—in all residential zones. In a stunning display of how the debate has been shifting, just yesterday Oregon legislators voted to legalize duplexes almost everywhere in the state, and fourplexes in all larger cities. Seattle’s sister cities Portland and Vancouver, BC, both have middle housing plans in the works.

    The Seattle Planning Commission has already laid out a path, writing that, “restoring the flexibility in housing types seen in Seattle’s historic residential neighborhoods is critical if the city is to achieve its goals of being a diverse, equitable and sustainable place to live.”

    *Note: Smaller cities that allow two ADUs per lot in single-family zones: Tigard, OR (pop. 50,000), Troutdale, OR (pop. 17,000), Fayetteville, AR (pop. 87,000), Langley, WA (pop. 1,000) , and Richmond, BC (pop. 198,000), and Sonoma County, CA (largest city Santa Rosa, pop. 175,000). Toronto allows more than one ADU per lot in its multifamily zones, but only one attached ADU per lot in its single-family zones.

    Thank you to Sightline Research Associate Nisma Gabobe for meticulous data mining to support this article.

    Dan Bertolet, senior researcher, writes and speaks about housing and urbanism. He spent ten years in urban planning, working with both private and public sector clients on projects that promote sustainable community development. Dan has a (healthy) obsession with the complex interplay between housing affordability, land use regulation, and transportation, and he has published extensively on these topics.  

    Margaret Morales, researcher, is a policy analyst. She earned her Master’s from The University of British Columbia’s Institute for Resources, Environment, and Sustainability where she was a Bridge Fellow focusing on issues of public health and the environment. She received her Bachelor’s degree from Duke University in Environmental Science and Policy, and English. She also holds a certificate in Documentary Video from Duke and worked on films for non-profits in the United States, Canada, and Latin America. 

    If you have questions or would like to make a media inquiry with Dan or Margaret, contact Sightline Communications Associate, Kelsey Hamlin.


    Sightline ADU research by topic

    ADUs and Climate

    ADUs and Affordability

    ADUs and Equity

    Political History and ADUs