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How ADU legislation could rein in Seattle’s McMansions

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Legislation to make backyard cottages more accessible could also prevent future massive single-family homes

Two homes are separated by a backyard lawn.
A backyard cottage.

While Seattle awaits a ruling from a hearing examiner about whether legislation to reform laws around accessory dwelling units (like mother-in-law apartments and backyard cottages), making them easier to build, can move forward, the Seattle City Council is preparing to do some fine-tuning. One less-publicized element of the legislation is a limit to floor-area ratio (FAR). This isn’t about the accessory dwellings themselves—it’s about the main home, and all homes in Seattle’s single-family neighborhoods.

If passed as planned, the legislation could reduce the prevalence of the McModern (that’s the modern version of the excessively large, cheaply built McMansion, as coined by Kate Wagner) in Seattle.

Simply put, FAR is about the building size in relationship to the lot size. For example, if a one-story home spread out over the entirety of the lot, the FAR would be 1. If a two-story home with an equal square footage on both floors took up half the footprint of the lot, it’s FAR would also be 1—but if that same two-story home took up a quarter of the lot’s footprint, it would be 0.5.

Single-family zoning in Seattle is currently extremely permissive. While it has height limits (25 to 30 feet), there’s no limit to FAR, as long as the giant home that results is only one home. This is how you get giant, out-of-scale houses in single-family neighborhoods: They’re built to maximize square footage under height limits.

The ADU legislation, in an effort to incentivize both building new homes and keeping older homes around, would set an FAR limit at 0.5 for the main home on a single-family lot, whether the home has ADUs or not.

According to city data presented in the city council’s Sustainability and Transportation Committee on Tuesday, homes built since 1990, but especially since 2010—including the now-infamous giant, boxy homes that creep straight up to the lot line—are far more likely to exceed this limit than older homes.

Homes above 0.5 FAR by year built, according to the city.
Seattle City Council

It could be a little while before the City Council passes legislation, said Seattle City Councilmember Mike O’Brien, who’s been ushering this legislation through the process for multiple terms now, and his committee wasn’t yet considering legislation. But the FAR requirements aren’t the most obvious piece of the policy.

“It was important for me to get this FAR regulation out there so we can get some feedback from homeowners to see if we’re missing anything there that we may want to consider,” O’Brien told Curbed Seattle.

A smaller FAR limit not only helps incentivize keeping existing buildings, but opens up space for ADUs without cramming everything inside the lot line.

The FAR limit would just apply to the primary home, explained O’Brien, and each ADU would have a 1,000-square-foot limit. “You could build a 2500-square-foot home with a 1,000-square-foot ADU inside the home so that would be 3,500 square feet, and then another backyard cottage of 1,000 square feet, so you could still get 4,500 square feet under this proposal on a 5,000-square-foot lot, but that would be three separate living units.”

“Additional units is what I’m hoping for, and I’d rather see more units than larger units,” O’Brien said. “Smaller units in general are going to be more affordable than larger units too, so it helps address affordability.”

The major idea behind ADU reform is around this affordability piece: ADU reform was among the recommendations of the Housing Affordability and Livability committee (HALA) in 2015, although reforms have been underway since 2014.

City Council staff, O’Brien said, is working on legislation for existing homes to not just be grandfathered in, but have some leeway for things like planned remodels.

One is similar to an exception carved out for existing homes in zones that went from single-family to residential small lot in recent rezones throughout the city: “If you have an existing ... one-story house and you want to make it a two-story house without expanding the footprint, even if adding that second story were to go over the FAR limit, that would still be okay,” said O’Brien.

“The other one would be a kind of a modest allowance if you wanted to expand—so you wanted to blow out a wall to build a larger kitchen or something,” O’Brien continued. That allowance would be something like 10 to 20 percent of existing square footage. A bigger house could also be accomplished, he added, by putting an ADU in your home.

“Both exemptions are just for existing houses,” he clarified. “So if you do tear down, no exemption, just build a small house.”

Other issues that could get some fine-tuning before final legislation include owner-occupancy requirements—the idea that to rent an ADU, the owner of the property must be living on it, too. The idea is to prevent speculative development that ends up creating unattainable housing. In the city’s preferred alternative in the environmental impact statement released in October, the owner-occupancy requirement is eliminated for the first ADU, but a minimum of one year of continuous ownership would be required for a second ADU.

“One of the things they were looking at were the impacts of a tear-downs of single family homes,” said O’Brien. The concern, O’Brien said, was that “developers will come in and tear down single family homes and build more new homes in backyard cottages.”

“What the analysis showed was, no, that’s not actually going to happen. It’s not a viable economic model ... but we are seeing a lot of developers tear down single family homes, and what they build is a bigger single family homes. And if we want to encourage folks to not do that and instead maintain existing family homes and build backyard cottages or in-law units.”

O’Brien said the committee will start considering legislation on May 28, at which point the council should have heard from the Hearing Examiner, and will start working with different possibilities, like parking and owner-occupancy requirements. The council will be holding a public hearing June 11, meaning the legislation could be voted out of committee on June 18—and possibly voted out of the full council by the end of June.