A peculiar architectural feature has spread throughout Seattle. On a single lot, you’ll see three houses, one somewhat bigger than the others, and between the big one and a smaller one is a walkway. Sometimes it’s on the ground floor, and sometimes it’s through the air — in other words, a skybridge.
On paper, what you’re looking at is a single-family home and two accessory dwelling units, an arrangement locally known as a 3-pack. These compounds popped up after Seattle eased building restrictions on A.D.U.s in 2019, as part of the city’s efforts to increase housing density and drive down prices. A.D.U.s are built on land that would not otherwise be developed — often, what would be a house’s backyard — and tend to cost less than conventional single-family homes.
Before 2019, Seattle allowed only one A.D.U. per lot, and the owner of the main house had to live on site and provide an off-street parking spot for any new unit. Under those restrictions, most A.D.U.s were built by homeowners on their existing lots, for use as guesthouses, studios or offices.
In 2019, the city removed the owner-occupancy and parking requirements and raised the number of A.D.U.s allowed per lot to two — but on lots containing two A.D.U.s, one had to be attached to the primary house. Developers, rather than individual homeowners, drove the next phase of A.D.U. construction, creating three-unit compounds to make the most of these unusual rules.
Alice Ji and her fiancé bought the main house in an A.D.U. compound in March. She said the skybridge connecting it to another unit is her laundry room, just big enough for two machines. “The developers did a really good job with soundproofing everything, so we don’t ever hear noise from our neighbors,” she said.
Bonnie McInnis, who bought an A.D.U. attached to a single-family home via their garages, said that she had originally looked for townhouses or condos, but that A.D.U.s in 3-packs were often a better deal. Her neighbors, she said, aren’t thrilled about the new construction and the increased density in the neighborhood. “They say they don’t like the units, but they still like the people who live in them,” she said.
The 2019 rules, including the attached A.D.U. requirement, represented a compromise. For years, City Council members had worked to introduce A.D.U. reform, and for years, neighborhood groups had sued to oppose it, fearing that increased density in residential areas would compromise “the very heart and soul of our city.” In that political environment, allowing for two detached A.D.U.s would have been a step too far, said Nicolas Welch, a senior planner at the City Office of Planning and Community Development.
Enter the skybridge. Some housing experts call them umbilical cords.
The New Townhouses
Under the building code, any enclosed structure wider than five feet can qualify as an attachment — “leaving room for interpretation,” Mr. Welch said.
“You write something down and it gets used in some creative ways that weren’t anticipated,” he added.
The 3-pack is a product of that flexibility. Developers have formed these A.D.U. compounds as three-unit condo associations, charging a nominal homeowner association fee (often $10 or less) to cover the filings. The largest unit, or main home, can sell for upward of $1.5 million, while the smaller ones, detached and attached, go for anywhere from $700,000 to $900,000 — in total, roughly commensurate with the sale of a three- or four-unit townhouse.
For years, most of the medium-density development had been in townhouses, said Trevor Johnson, a founder of Blackwood Builders Group, a local real estate development company. But that began to change after the new A.D.U. rules took effect.
In 2019, fewer than 300 A.D.U. permits were granted. But nearly 500 were granted a year later, about 1,000 in 2023, and about 900 in 2024. Mr. Welch estimated that somewhere between one-half and two-thirds of the new A.D.U.s appeared to be built by developers.
Another law passed in 2019 played a role in the A.D.U. boom: the city ordinance establishing the Mandatory Housing Affordability program, which upzoned large swaths of the city’s neighborhoods. The program requires developers building in certain zoned areas to either include affordable housing on site or pay into a city fund for building affordable housing.
Many of the areas zoned for townhouses became subject to M.H.A. program rules, while many of the areas zoned for single-family homes — and ripe for A.D.U. construction — were exempt from the program. The difference in fees could be tens of thousands of dollars, Mr. Johnson said, a big deal to smaller developers like him.
Some of the construction logistics — like sharing a water meter between the attached A.D.U. and the main structure instead of installing separate meters — also led developers to favor A.D.U. compounds over townhouses. “They’re just simpler,” as Mr. Johnson put it.
‘A Layout to Fit Every Lifestyle’
On real estate sites like Zillow and Redfin, the new A.D.U.s are typically marketed like any other single-family home, often boasting privacy, tranquillity and luxury; photos rarely show the connection points. But sometimes a listing will offer a choice of three units in a compound — “with multiple floor plans to choose from, there’s a layout to fit every lifestyle,” one read.
Designers have tried their best to make the connection points functional. In some compounds, first-floor hallways are vestibules connecting the backs of houses that face in opposite directions; in others, they’re entryways or garages.
Shawna Ader, a real estate broker at Windermere, is working with a client to sell a home attached to an A.D.U. where the hallway connection is the A.D.U.’s mudroom. “There’s double wall construction as a sound barrier,” she said, “but it’s not a space you’re ever really spending time in.”
In some homes, the connection points are living spaces. Mr. Johnson lives in the main house of an A.D.U. compound and uses the skybridge as an office. He rents out the attached A.D.U., but plans to sell it; he recently sold the detached unit.
Despite the thoughtful design, Ms. Ader said, many buyers remain skeptical of A.D.U. compounds. “We’re seeing this model more and more, but it hasn’t really caught on,” she said, noting that some clients rule out 3-packs outright, preferring single-family homes. “We all want more density in this city to give people more housing options, but right now, it’s like dragging horses to water to make them drink.”
And the process of moving into a 3-pack can come with unique challenges. Ms. McInnis, the attached A.D.U. owner, said that she and the owners of the main house in her 3-pack needed to purchase a joint home insurance policy that would cover their shared wall, but they had a tough time finding a company to take them on as clients. The Postal Service and Comcast also initially had issues dividing up services among the three units, she said.
Has It Worked?
Seattle’s housing market is one of the most expensive in the country, with a median home price of $900,000 in 2024, according to the Washington Center for Real Estate Research. Since the tech boom of the 2010s, demand for housing in the city has far outstripped supply. A.D.U.s tend to help with these pressures in two major ways, by being less expensive than the single-family homes they share lots with and by increasing the amount of housing available, said Emily Hamilton, director of the Urbanity Project at the Mercatus Center at George Mason University.
The skybridge A.D.U. boom certainly added to the city’s housing stock, but it’s not clear whether it meaningfully increased the overall amount of affordable housing. With prices in the high six figures, the new A.D.U.s are not what most people would consider affordable (though they may meet the legal definition based on area median income).
Alice Poggi, a longtime resident of Phinney Ridge, a North Seattle neighborhood that has been a hotbed of A.D.U. construction, worried about how the new development was changing her neighborhood. “Our properties are now commodities with developers scrapping these smaller homes and building three units per lot, covering yards with three-story boxes, losing legacy trees, and none of it being affordable,” she said. “Our properties are losing the family housing model that defines neighborhoods.”
In many cases, it seems, developers that might have built other housing, like townhouses, have just turned their attention to A.D.U.s, which offer more bang for their buck.
Developers may also have changed where they built more than how much. A recent analysis of the city’s M.H.A. program found that there were fewer building permit applications of all kinds in areas that were subject to M.H.A. rules than in areas that were not — mirroring the shift from townhouses to A.D.U.s that Mr. Johnson, the developer, described.
Jacob Krimmel, a co-author of that study and a senior economist at Realtor.com, said that developers told him the combination of new A.D.U. and M.H.A. regulations had led them to change their construction strategies. “If upzoning is the carrot and M.H.A. fees are the stick, it appears developers are just saying, ‘Let’s move to an area where we don’t have to worry about carrots and sticks at all,’” Mr. Krimmel said.
That regulatory environment is set to change again, as the city moves to comply with two state laws passed in 2023 that require it to allow two detached A.D.U.s, as well as duplexes, triplexes and fourplexes, on all residential lots. Those laws, which take effect June 30, may spell the end of skybridge and 3-pack development in Seattle, though these architectural relics will continue to mark the city’s neighborhoods for years to come.
Urban planning and policy experts will be watching the transition. Seattle is one of the nation’s “policy labs” for new housing regulations, Mr. Krimmel said. “What works well and what hasn’t informs the national conversation, as many metro areas are confronting the same issues.”
Ruth Fremson is a Times photographer, based in Seattle, who covers stories nationally and internationally.
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