HOMEOWNERSHIP HAS TRADITIONALLY been a marker of middle-class life, and a tool for vaulting families solidly into the middle class. Sustainable homeownership builds family wealth, and helps stabilize neighborhoods. But over the past decade, the nature of homeownership in Massachusetts has shifted drastically.

Middle-class families in Massa-chusetts are increasingly struggling to access homeownership—especially in greater Boston, where the bulk of the state’s jobs are located.

The past 10 years have seen a sharp erosion in middle-class homeownership across Massachusetts. There are now far fewer middle-class homeowners in Massachusetts than a decade ago, and the average Massachusetts family can no longer afford to buy the average Massachusetts home.

This shift is deeply concerning, because it challenges long-held assumptions about what homeownership looks like in the Commonwealth, who it’s for, and what role homeownership plays in the state’s economic life. Without a concerted policy response to increase home production in Massachusetts and bring down home prices, homeownership is in danger of becoming an upper-class luxury.

The number of owner-occupied homes has essentially been flat for the past decade: Massachusetts has 1.6 million owner-occupied homes, but the state added just 12,000 home-owning households between 2005 and 2015, the most recent year for which Census data are available.

This overall flatness in homeownership masks some seismic shifts, though.

Massachusetts lost roughly one in every four lower-income home-owning households between 2005 and 2015. That is, Census data show that the number of homeowners with household incomes under $50,000 has shrunk by nearly 25 percent over the past decade.

Tim Sullivan, the executive director of MassHousing.

Additionally, over the last decade, the state lost nearly one in every five of its middle-income homeowners—those with household incomes between $50,000 and $100,000. In 2005, two-thirds of Massachusetts homes were occupied by households earning below $100,000; that share has now fallen to slightly more than half.

At the same time that middle-income and moderate-income homeownership are being crowded out of the housing market, Massachusetts has seen a dramatic homeownership expansion at the upper tier of the income spectrum. Nearly all the state’s growth in homeownership is happening among the top 20 percent of wage-earners, and among those households with incomes that are at least twice the state’s median wage.

This sorting of homeownership by income is a national phenomenon, but the swing away from moderate- and middle-income homeownership has been far more exaggerated in Massachusetts than in the country at large.

The growing inability of working Massachusetts families to afford homeownership—reflected in the shrinking ranks of moderate- and middle-income homeowners in the Commonwealth—is a symptom of this state’s longstanding housing production dysfunction.

Massachusetts has struggled, for decades, to deliver the kind of housing residents want to live in, in the quantity residents demand. The pace of overall housing construction is now roughly half what it was during the 1960s and 1970s.

Because Massachusetts is a home-rule state, where each city and town controls its own pace of development, housing growth has been extremely uneven. A handful of fast-growing communities are shouldering much of the Boston region’s growth. Data compiled by the Metropolitan Area Planning Council show that, over the past decade, more than half of the multi-family housing construction in Massachusetts has occurred in just five communities—Boston, Cambridge, Chelsea, Everett, and Watertown—while 207 of the state’s municipalities did not permit any multifamily housing development.

The planning council estimates that by 2040 cities and towns in the 101-municipality metro Boston region will need to add up to 435,000 new housing units to keep up with housing demand. Two-thirds of this future housing demand is for multifamily housing, appropriate for younger residents, young families, and empty nesters.

Massachusetts already has a significant stock of single-family housing, and the Metropolitan Area Planning Council estimates that two-thirds of the future demand for single-family homes can be met with existing homes. But this math only works if the households who want to live in smaller townhomes and town centers have access to the type of denser, more walkable housing they demand. Cities and towns aren’t currently building enough new multifamily housing to keep up with demand. As a result, the gears of the housing market are getting gummed up.

When families aren’t able to cycle between the types of housing they demand, households get locked in place, and a shortage of homes for sale drives up prices. This is happening right now, across Massachusetts. The inventory of homes for sale is historically low, and has been declining for five straight years. For-sale inventory over the five years has been falling, and home prices have shot up by 30 percent.

Slow multifamily housing development is compounded by the large-lot, single-family zoning that’s common across much of Massachusetts. The average lot required by local zoning is far larger in Massachusetts than across the rest of the country. Large-lot zoning promotes suburban sprawl, and creates perverse incentives for homebuilders to build large, expensive homes that are unaffordable to the average family. Nationally, homebuilders are building far fewer starter homes than they did even a decade ago; minimum lot sizes in Massachusetts exceed what zoning across the rest of the country requires, making it even more difficult to build modestly-sized, modestly-priced starter homes.

Restrictive zoning keeps the supply of new housing from meeting regional demand. These restrictions have caused the price of housing in the Commonwealth to spike: since 1980, home prices have risen twice as quickly in Massachusetts as they have in the rest of the US. As a result, moderate- and middle-income homebuyers are increasingly being pushed to the sidelines. Research released last year by the Urban Land Institute and sponsored by MassHousing found that very few towns in eastern Massachusetts remain affordable to middle-income families. Data from the Metropolitan Area Planning Council show that fewer than one in every three home sales between 2014 and 2015 in metropolitan Boston were affordable to an average family.

It is not economically sustainable to have substantial homeownership growth only happening at a level that’s at least twice the median income, while the broad middle class is crowded out of the housing market. If housing price burdens fall heavily on the Massachusetts workforce, and particularly on recent college graduates and young families, the state won’t be able to continue to attract the educated, talented workforce needed to grow the economy. Highly-educated workers are mobile, and many of the Bay State’s economic competitors already offer broader housing choices, at more affordable prices.

Housing affordability challenges also lead to more income inequality. Last year’s Urban Land Institute report found that eastern Massachusetts has been steadily losing middle-class residents, while gaining residents at both extremes of the income spectrum. By pricing middle-class buyers out of the housing market, high housing prices contribute to the hollowing out of the region’s middle class.

Policy decisions created Massachusetts’s middle-income affordability crunch. And strategic policy responses can ease it, and create meaningful homeownership opportunities for residents across the Commonwealth. Here are three places for policymakers to start.

Create new opportunities for suburban multifamily construction: Boston can’t carry all of the Common-wealth’s new housing supply, and there’s strong demand for new suburban apartments, condominiums, and townhomes for residents of all ages. Cities and towns outside Boston can help meet this demand, and strengthen their own footing, by thinking creatively about how to harness new housing to drive broad-based community development.

For instance, MassHousing is partnering with Beverly to transform a vacant MBTA parcel at the Beverly Depot commuter rail garage into a mix of market-rate and subsidized housing for those who don’t qualify for traditional affordable housing but who still cannot afford market rents. At a groundbreaking ceremony earlier this year, Beverly Mayor Michael Cahill argued that the new downtown housing would improve business for the city’s small retailers. Similar initiatives are taking place in Medford and Plymouth.

In each of these cases, municipalities are growing their housing stock to boost local tax revenues, create new jobs, and create a greater diversity of housing options for their residents. And these types of opportunities—to promote healthier town centers, and to revitalize tired retail strips through the development of new housing—exist across the state.

Build more starter homes for young families: To increase middle-class homeownership, Massachusetts needs to create homeownership opportunities that are affordable to middle-class buyers. The zoning in many communities actively works against that outcome, though. The mature suburban streets that exist in communities across Massachusetts likely wouldn’t be able to be built today, thanks to decades of down-zoning and today’s large-lot requirements. In Falmouth, for instance, a developer looking to replicate the single-family development pattern adjacent to his project site is pursuing building permits under Chapter 40B, the state’s anti-snob zoning statute; current zoning only allows for the construction of three new homes, on a site that contains 4.6 acres of buildable land.

Communities now have an opportunity to break this pattern, and encourage homebuilders to deliver single-family homes affordable to first-time homebuyers. In last year’s economic development legislation, Gov. Charlie Baker secured passage of a new incentive under the Chapter 40R smart growth law to encourage the development of smart growth starter home subdivisions. By adopting new overlay zoning that promotes the development of modestly-sized homes on quarter-acre lots, communities can now provide an alternative to large-lot sprawl. By shrinking lot sizes and home footprints, this new zoning will put downward market pressure on new home prices.

Promote sustainable borrowing: As the state’s affordable housing bank, MassHousing finances homeownership opportunities for low-, moderate- and middle-income homebuyers in Massachusetts, on behalf of the Commonwealth. MassHousing makes homeownership possible to residents, including many first-time homebuyers, by offering conventional mortgages with low down payments and affordable rates. Last fiscal year, MassHousing financed $662 million in total homeownership lending, including $197 million in mortgages for low- and moderate-income buyers (with incomes 80 percent of the area median income), and $232 million in lending to 1,100 buyers in the Gateway Cities.

This work—helping working households attain long-term, sustainable homeownership—is a core component of MassHousing’s mission-driven work. It’s even more consequential today, with broader forces pulling against it.

Tim Sullivan is the executive director of MassHousing, an independent, quasi-public agency charged with providing financing for affordable housing in Massachusetts.