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All for one, and none for all: Is banning single-family zoning easing the housing crisis?

There is nothing wrong with them, but not everyone can afford one.

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Cities and towns wield a powerful toolto control their built environment, dictating what can go where within their borders: zoning. It’s why you can’t just open up a laundromat in your garage — some areas are zoned for commercial, industrial, or mixed uses, while others are set aside for residential homes.

And in most communities around Greater Boston, more than 80 percent of the available land is zoned solely for single-family homes, according to the Fair Housing Center of Greater Boston. That makes it illegal or next to impossible to, say, convert a finished basement into a rental unit or to build even modestly sized multifamily dwellings, like the region’s familiar three-deckers, in much of suburbia.

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There’s nothing wrong with single-family houses, of course. But not everyone wants (or, more to the point, can afford) to live in one. In fact, not even half of Boston-area households live in a detached single-family home, according to American Community Survey data. And by restricting nearly all residential development to the postwar ideal of a freestanding home on a large — and therefore expensive — lot, communities can essentially keep many lower-income people out, maintaining patterns of housing segregation that formed in the early and mid-20th century.

“Single-family zoning wasn’t created as a means to exclude, but it very quickly kind of became a means of exclusion,” said Christopher Ptomey, executive director of the Urban Land Institute’s Terwilliger Center for Housing.

Simply put, single-family zoning rules — or “apartment bans,” as they’re sometimes called — are a big reason housing is so scarce, expensive, and exclusive in certain places. “Zoning for single-family houses has been … one of the major contributing factors to the crisis of housing supply and affordability,” said Jessie Grogan, associate director of reduced poverty and spatial inequality at the Lincoln Institute of Land Policy in Cambridge.

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By continuing to outlaw more affordable housing types — namely condos, town homes, and rental apartments — “single-family zoning today makes it more difficult to address the equity problems it’s caused historically,” Ptomey said. That has led housing advocates, economists, and lawmakers to ask the same question: If single-family zoning helped create our affordable housing shortage, and tends to perpetuate historic patterns of inequity and segregation, why not get rid of the cause?

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The first cities through the wall, so to speak

That’s why, in December 2018, Minneapolis effectively ended single-family zoning, allowing duplexes and triplexes to be built citywide. That doesn’t mean an end to single-family homes, or that quaint bungalows will soon sulk in the shadows of towering apartment buildings. It simply allows homeowners to build a two- or three-family residence on their lot without having to go through the onerous and costly process of applying for a zoning variance. The state of Oregon followed suit in 2019, and this past fall, California — the epicenter of the nation’s affordable housing crisis — legalized the construction of duplexes, triplexes, and fourplexes on most single-family lots statewide.

The idea is to add extra housing supply gently and gradually to existing neighborhoods — supply that should, according to basic economics, help meet some of the demand for homes and therefore ease price growth.

So have those efforts yielded more housing or made a dent in affordability? The short answer is: not really, not yet. But experts say it’s much too soon to expect dramatic results from such a long-term strategy. “It’s going to take years before we see the number of units being added that will really make a dent in affordability, but it’s a step in the right direction,” said Redfin deputy chief economist Taylor Marr. “It’s sort of a necessary but insufficient piece of regulation.”

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“It took 30 or 40 years to see the impacts of what happens when you zone so much land for single-family use. And really, it’s going to take 30 or 40 years for us to see how successful we’ve been in undoing that,” Grogan added.

That’s not to say there haven’t been promising signs. In the 10 years from 2009 to 2018, Minneapolis permitted a total of 42 duplex units, according to the Department of Housing and Urban Development — about four per year. But from 2019 through October 2021, builders already have permitted 80 duplex units, more than a sixfold increase in the average annual rate and about four times as many as in nearby St. Paul. The 30 duplexes permitted in the city in 2020 were the most since 2003 and could soon be surpassed by the 2021 total.

Still, a few dozen duplexes do not a housing crisis solve. Ptomey agreed that “upzoning” more single-family districts is just one small but much-needed housing policy. “In real estate, there’s a notorious amount of risk aversion, and I think it’s going to take a while — and for some property owners to start doing this — for others to kind of see the potential and to feel comfortable and safe changing the properties they own in this way.”

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That’s how it’s played out with accessory dwelling units (ADUs) in California, which legalized the development of so-called in-law apartments on most residential lots in 2017. It took time for people to get comfortable with the concept and the process, Ptomey said, but through 2020, California had added more than 22,500 accessory housing units in three years, according to the University of California Berkeley, with thousands more permitted. “I think you’re going to probably see the same thing over time in these areas where you can have duplexes and triplexes,” Ptomey said, “a similar kind of slow growth on the front end, but then as people see the opportunity, it will build on itself.”

“Single-family zoning wasn’t created as a means to exclude, but it very quickly kind of became a means of exclusion,” said Christopher Ptomey, executive director of the Urban Land Institute’s Terwilliger Center for Housing.Adobe Stock/tamas - stock.adobe.com

Preserving a neighborhood’s character

In suburban swaths of single-family houses, existing homeowners often bristle at the notion of adding density, fearful it will change the character of their neighborhood. But in places like Ipswich and Sudbury, density often predated sprawl and culs-de-sac by several centuries.

“In a lot of those suburban towns, you already have some pretty good examples of what [upzoning] looks like as you get close to the town center,” Grogan said. “To some extent, having more density, and having town houses and apartments above retail, is a more traditional form of development than single-family houses on large lots.”

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“There is a way to do density that fits with the character of all different kinds of places,” she added. “That looks different in Somerville than it does in Lincoln, but it does not have to mean you’re eliminating single-family zoning and all of a sudden everybody’s building 10-story buildings.”

Another common fear among single-family homeowners is that allowing other housing types to be built will hurt their property values. In fact, the opposite is often true. “They worry, ‘Oh, you’re trying to improve affordability, that means you want to make my house value decline.’ And the two are not at odds,” Marr said. “What just happened was that the development value of your land just got much greater, because the potential of that land has increased.”

In Minneapolis, existing single-family homeowners in upzoned areas saw their property values increase 3 percent to 5 percent compared with similar lots in neighboring communities, according to early research by Daniel Kuhlmann, assistant professor of community and regional planning at Iowa State University.

The math is simple: Imagine a half-acre suburban lot outside Boston that holds a single million-dollar house. If the land were upzoned to allow greater density, that half acre could comfortably fit the original house, plus a pair of town houses, which would sell closer to the median price of a Boston-area condo, around $625,000. Suddenly, the same plot of land has become more valuable while also creating more affordable housing options.

One benefit of upzoning residential neighborhoods, as opposed to building new homes on greenfield sites (land that has not been built upon previously), is that it leverages existing infrastructure. “The roads are already there, and a lot of Greater Boston’s neighborhoods are in walking distance of historic downtowns, village centers, and public transportation,” said Amy Dain, a housing policy consultant in Newton.

It also tends to add density in a subtle, gradual way that’s less disruptive to a neighborhood’s feel than, say, an 80-unit apartment complex. Even if just one out of every 100 single-family homeowners in Massachusetts added a second unit, it would net us more than 15,000 new housing units statewide.

“If you’re talking one in 100, or even one in 20, those are really invisible,” Dain said. “It doesn’t really change the character of anything — they’re highly dispersed, they’re not causing major traffic impacts, parking problems … history shows us that not everybody’s going to turn their single-family house into a duplex.”

Apartment bans in wealthy suburbs don’t just make it harder for low- and middle-income people to live in those communities; the same exclusionary zoning practices also shift development into other areas, leading to gentrification and displacement in historically low-cost neighborhoods that welcome the new construction, Dain said.

“The root cause of the escalation in home prices is not the new cafe, the new apartment building, or the new neighbors — the root cause is a region-wide housing shortage, [which] creates a game of musical chairs that people with the lowest incomes will lose,” Dain said. “In the absence of restrictions, we would expect to see more development in the most expensive places,” Dain added, where the profit potential is higher for builders. But in Greater Boston, she said, there’s more growth pressure in areas like Everett or Lynn than Winchester or Marblehead. “Homeowners in a hundred suburbs are using local laws to prevent the market from building enough housing. The zoning laws in Wellesley, Belmont, Winchester, Needham, Westwood, Milton, Cohasset, Weston, and every exclusive community, are causing displacement in Roxbury, Hyde Park, Medford, Revere, Quincy, and so many places.”

Single-family zoning is just one arrow in the quiver for apartment-averse communities; other exclusionary rules, such as minimum lot sizes, setbacks, and parking requirements, yield similar results by different means. The best indicator that recent single-family zoning reform efforts are working, Ptomey said, is that people have begun discussing changes to those other policies, too.

Grogan likens the challenge to scaling Mount Everest. “One policy change, or building in one kind of place, is not going to get us all the way to the top. It’ll move us up the mountain, but we need to layer all of these efforts … building different kinds of housing units in different kinds of places, if we’re really going to deal with the magnitude of our affordability crisis.”

Jon Gorey blogs about homes at HouseandHammer.com. Send comments to jongorey@gmail.com. Follow him on Twitter at @jongorey. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.

Correction: Because of a reporting error, an earlier version of this story included the wrong figure for the net number of homes that would be created if 1 in every 100 single-family homeowners in Massachusetts added a second unit. The Globe regrets the error.