You’ve probably watched it happen in your Seattle neighborhood — workers demolish a modest bungalow with a garden, then cram the lot with a bulky new house shaped like a box.

City Councilmember Mike O’Brien has seen it, and he wants to stop it from happening so often. His proposed “McMansion” ban would outlaw the type of large new homes that many neighbors abhor.

The council member is championing legislation that would ease restrictions on backyard cottages and parent-in-law apartments while at the same time tightening restrictions on large, detached single-family houses.

In theory, the proposal would encourage property owners to build new houses with accessory units, add accessory units to existing homes or just do nothing. At the same time, the large-house ban would take away a popular home-building option.

O’Brien’s proposed limit would have blocked almost half the new single-family houses constructed in Seattle since 2010, had it been on the books.

“With most new houses, especially those built speculatively to sell on the market, the trend has been to build as big as you can,” said Boyd Pickrell, an architect with the Seattle-based development company Green Canopy.

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O’Brien has described backyard cottages and basement apartments as a gentle way to add density as Seattle’s population grows, knowing he can count on urbanists as cheerleaders. By taking aim at McMansions, he hopes to also win support from residents grumpy about development.

“You see people who tear down a house and build a larger house that may be out of scale and that doesn’t add any housing,” the council member said. “They’re replacing a less-expensive housing unit with a more-expensive housing unit. That’s taking us in the wrong direction.”

The changes related to accessory units have been in the works for years and have received intense scrutiny. Earlier this month, Seattle beat a legal challenge brought by a neighborhood group with concerns about O’Brien’s push to allow more cottages on more lots across the city while removing requirements for off-street parking and for property owners to live on site.

But the council member’s plan to limit large houses has received less attention. That could change in the coming weeks, because the McMansion ban would dramatically alter Seattle’s home-building rules and could help determine what the city’s neighborhoods look like decades from now.

Under O’Brien’s legislation, which his sustainability committee will begin discussing Wednesday, the city would adopt a maximum Floor Area Ratio (FAR) of 0.5 for new houses on most lots. For example, a new house on a 5,000-square-foot lot could have no more than 2,500 square feet of above-ground living space for a single family (not including accessory-unit space). A new house on a 6,000-square-foot lot could have no more than 3,000 square feet of single-family space, and a house on a 7,000-square-foot lot could have no more than 3,500 square feet.

For new houses on lots smaller than 5,000 square feet, there would be a cap of 2,500 square feet, rather than an FAR limit.

Only 9.5% of Seattle’s existing single-family houses would have been blocked had O’Brien’s proposal been in place when they were built.

But 47% of new houses built in the city since 2010 (1,753 of 3,694) would have been blocked, according to data requested by The Seattle Times.

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Some are actual mansions on sprawling lots, while others are somewhat large homes on small lots. None would have been allowed under O’Brien’s plan.

“The numbers really tell the story … when people are building new houses, they’re building these huge structures a lot larger than the typical structure,” the council member said. “That reinforces the urgency around this policy.”

Many recently-built houses that would have been blocked are in neighborhoods relatively close to downtown, where lot sizes are smaller. Few are in Seattle’s northernmost neighborhoods, where lots sizes are larger.

With the accessory-unit changes and McMansion ban, Seattle would add 4,430 accessory units and see 1,580 houses torn down over 10 years, according to a city analysis. Without the changes and the ban, there would be an estimated 1,970 units added and 2,030 houses torn down.

Different views

Some developers are opposed to O’Brien’s ban. Though Keith Hammer is a proponent of loosening restrictions on accessory units, he worries the FAR limit would cause him to lose business in the short term. Hammer can’t recall the last time his nwBuilt company constructed a Seattle house smaller than 2,500 square feet.

“In some neighborhoods, the status quo is a larger house. That’s what people want to buy in some neighborhoods,” he said. “I’ve been building 3,000- to 3,500-square-foot houses … I wouldn’t be able to do that anymore and that product would no longer be available to the public.”

There could also be unintended consequences, Hammer said. Some people would likely build a large house with a basement unit and then occupy both, rather than renting. Meanwhile, construction costs would increase.

“Maybe multiple generations want to live together. You’d be making that more expensive for them,” Hammer said. “Building below grade costs more.”

Another developer is more optimistic. Sam Lai is excited about working on more houses with accessory units, he said.

“The outcome is probably going to be more units being built and less square footage per occupant, which would be great from an environmental perspective,” said the Green Canopy co-founder, whose own property includes an accessory unit. “It also would provide us with more options in terms of what we could design and build for prospective buyers.”

There could be huge latent demand for houses with accessory units, Lai said. Developers don’t really know because bank loans for such projects have been hard to come by, he said.

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Mayor Jenny Durkan has said she wants to see more accessory units constructed. She asked the Seattle Department of Construction and Inspections last year to pre-approve certain backyard-cottage designs.

Durkan hasn’t directly addressed the owner-occupancy question, nor the McMansion question. Skeptics have predicted that removing Seattle’s owner-occupancy requirement for lots with accessory units could lead developers to replace older houses with multiple rental apartments.

The mayor wants to “open all neighborhoods to more diversity,” but she also wants to “ensure we do not encourage speculation,” spokeswoman Kamaria Hightower said in an email.

The Durkan administration hosted several invitation-only meetings this month “to hear from residents about ways the city can better support homeowners” who want to build accessory units, Hightower said. The meetings were in the Central District, Ravenna and West Seattle, and on Beacon Hill.

The council will host a public hearing June 11, O’Brien said. His committee could vote on the legislation as soon as June 18 and the council could adopt the plan as soon as June 24.