Housing costs have bedeviled cities and towns for years, but the problem is fast becoming a crisis—especially in places driving America’s economic growth, like San Francisco and Boston. Since 2011, home prices nationwide have risen more than twice as fast as household incomes. When taxpayers have to subsidize rents just so city employees can live where they work and Facebook starts getting into the homebuilding business, something isn’t right.

Sonja Trauss has a simple solution: Build. Frustrated by sky-high rents in San Francisco, Trauss, then a high school math teacher, started attending local planning meetings in 2014 to advocate for more construction and taller buildings. That led her to co-found the YIMBY Party—short for “yes in my backyard”—an advocacy organization that today has 600 dues-paying members. What started as a small act of civic-mindedness has grown into a national movement. The YIMBYs are also making their mark in Austin, Denver, New York, Washington and other cities, where activists are pushing for denser development and challenging old rules, like parking requirements that take up valuable real estate. Last year, YIMBY crusaders held their first national conference.

Source: U.S. Census Bureau American Community Survey

If it seems strange that a grassroots movement would need to arise to help developers build, that speaks to the near-stranglehold that towns and neighborhood groups enjoy over homebuilding permits. YIMBYism emerged in reaction to not-in-my-backyarders—NIMBYs—who became fixtures at city halls during the go-go building boom of the 1980s and 1990s, using the levers of environmental protection, historic preservation and municipal zoning rules to prevent new construction and keep property values up. It worked, perhaps too well: Anti-density activists have helped send housing prices skyrocketing in many cities. Now Trauss & Co. essentially provide political cover to local leaders to approve big projects—and they are starting to see results. Last year, Trauss helped persuade San Francisco to loosen certain zoning restrictions and campaigned, successfully, for a pro-housing California state senate candidate. In October, YIMBYs outnumbered NIMBYs at a local planning meeting for the first time.

It’s a start. For now, Trauss and her boyfriend pay about $3,000 a month for a 650-square-foot apartment in San Francisco. Their first child is due in November. “I hope this movement is a flash in the pan,” she says. “I don’t want to still have this problem in 20 or 30 years.” —Lorraine Woellert

Q & A Best book you read this year? Roxana, by Daniel Defoe Out-of-the-box policy idea that we aren’t paying enough attention to? Eliminating childhood (and adulthood) lead exposure. Twitter: Useful new tool or obnoxious distraction? Useful! Is Trump draining the swamp? No. What historical moment does 2017 most resemble? The last wave of urbanization, the end of the 18th century / first half of the 19th century.