Some political leaders also emphasized the role of single-family zoning in creating and perpetuating racial and economic segregation. They said it was time to chip away at that legacy. And while opponents warned about flooding and erasing green space, there is a strong environmental case for increased density, too: Adding homes in developed areas limits driving, and the development of outlying areas.

But in Montgomery County, as in most areas slowly moving to allow more development, political leaders sold the changes by emphasizing they were taking a small step that was unlikely to disrupt the status quo. “This is not going to be a humongous proliferation,” said the Council president, Nancy Navarro. Opponents concerned about more density were told that it would remain about the same. Those worried about living near poor people can find comfort in the fact that there won’t be enough new units to cut housing prices.

The problem is that places like Montgomery County need big, disruptive changes to provide affordable housing for a growing population, to address economic and racial inequality and to limit climate change. The county, which has more than one million residents , a larger population than all but a handful of American cities, is increasingly unaffordable, with average housing prices soaring above $400,000. The county’s employees are among those looking in from the outside. A county survey last year found that 45 percent of Montgomery County’s 9,243 employees lived elsewhere. Richard Hoye, 63, a retired firefighter and county resident, said that included most of his former colleagues, and was more common among younger firefighters. “Why should somebody risk their lives in a county where they can’t afford to live?” he said.

(Mr. Hoye bought a house with help from his parents in the early 1990s. He supported the new law because he said it would let him continue living in the county, by converting his basement into an apartment where he can live in retirement.)

Ms. Navarro pointed to a recent regional study estimating that Washington, D.C., and its suburbs need nearly 26,000 new housing units each year to keep pace with job growth — a goal that the region has not come close to achieving at any point in the last 10 years. The anger stoked by a minor leniency makes clear how hard that goal will be.

One county resident, Katherine C. Gugulis, wrote a protest letter in The Washington Post that concluded, “Just because others flee crime-ridden and poverty-stricken areas doesn’t mean Montgomery County has to be turned into a slum to accommodate them.”

Aleksandra Rohde, a retired Army lawyer, described living in the county as a privilege that people need to earn. “You work hard, and you get to go live in Montgomery County,” she said. “Does that mean it has to be the first place you live? No. You can drive a little further and work your way up to it.”