America isn't building enough homes. A new map may fix that.

Depending on who's doing the estimating, America is short anywhere from 2 million to 6 million homes. If you've tried to buy or rent a place in the past year, or you know anyone who has, then you didn't need me to tell you that. Hardly anything's on the market, and none of us can afford what is. The question is: Why?

That simple question, oddly, has been impossible to answer with any real precision. The housing shortage may be national, but the problem is local. Where homes get built, how many, what type, how big the lot has to be, how many meetings it takes to build something new — those things are all governed by zoning rules. And every town and village and city zones itself. Which means there are more than 30,000 different sets of zoning rules in America. When it comes to housing, we're a nation of islands, governed by no central authority.

To make matters worse, each of those 30,000 islands has its very own language for those rules. Many maps favor the suburban ideal of single-family homes on expansive lots, while essentially disallowing every other form of housing. One place might call a two-family home a duplex (and allow it), where another code might call it a "townhome" or just "multifamily" (and nix it). In some places, "mixed-use" means a neighborhood that combines homes and shops (nope!); in others it means a combination of offices and industry (sure, why not?). Some cities have maps digitally coded into a standard geospatial data format; other towns still use paper. In city after city, the rules are woefully outdated, head-scratchingly obscure, or outright racist.

If you could decipher all those rules, and make comparisons between different cities and states, you might be able to figure out which rules let more homes get built, and which ones don't. But no one has ever assembled America's zoning regulations in one place, let alone force them to use the same words to mean the same things — until now.

For the first time, a team of researchers is compiling every city's zoning rules into a National Zoning Atlas. That means everyone from policymakers to homeowners will be able to look at their local zoning maps and understand their town's hidden architecture. And maybe, armed with that information, we'll finally be able to remodel America's fixer-upper of zoning policies into a tasteful showcase for starter homes and cheap apartments. 

"Zoning is hugely influential on all of our lives, and people don't know enough about it," says Sara Bronin, an architect and attorney at Cornell University who founded the atlas. "Our project really aims to demystify these hidden rules, and encourage policymakers, researchers, and advocates to mine that information."

The atlas also aims to translate the nation's multitude of local zoning rules into what Bronin calls a "common set of definitions and practices" — a rationality that will enable analysts, at long last, to make "apples-to-apples comparisons." In other words, we'll finally be able to say, with certainty, which policies build more homes and bring prices down, and which policies don't.

It's still early — so far the atlas includes only 2,000 jurisdictions populated by 35 million Americans. That's because it's more than just a massive data project, crunching code into computers. "It's a legal research project," Bronin says, "one of the biggest legal research projects on local government law that has ever existed. We are effectively reading hundreds of thousands of pages of local ordinances and regulations."

It's hard to underestimate how powerful a tool the atlas will be. Bronin says people are already using it to effect policy changes — like the woman in Milford, Connecticut, who printed out screenshots from the atlas to persuade her town to change its policy prohibiting residents from building guest-house-type units in their backyards. Milford's official zoning map was an incomprehensible mosaic of microscale neighborhood rules — but the easier-to-read atlas enabled the Milfordites to prove their case, and demand change.

Or look at the even bigger changes that were enacted in Montana, where housing prices spiked sharply during the pandemic as more people moved to Big Sky Country from out of state. Armed with the apples-to-apples zoning maps contained in the atlas, advocates for zoning reform were able to compare the rules in Montana with those of California. Cities and towns in both states, they could see, penalize or outright prohibit duplexes and other forms of housing that bring down prices and help prevent urban sprawl.

"We were able to come into legislative hearings and say: Here's a map of Missoula's zoning, here's where you can build duplexes, and here's where you can't. And then we'd put a map next to it of Los Angeles' zoning, and they looked identical," says Kendall Cotton, a prime mover behind the push for zoning reform. "We said: Look, if Missoula zones like LA, it's going to grow like LA. And in 25 years, it's going to look a lot like LA."

The beauty of that argument is that it appealed to both sides of the political divide — and not just because everybody came together over their shared hatred of Los Angeles. Enacting a more permissive set of zoning rules gave everyone something they wanted. Lefty political leaders knew that sprawl fuels climate change and that housing shortages exacerbate inequality. Right-leaning folks hoped to preserve their right to build whatever they wanted inside their own property lines.

"When we put it on a map, it was undeniable," Cotton says. "They had to face facts and admit that what their zoning code represents is not conducive to building all the homes they're going to need." Montana passed new rules to accommodate smaller multiunit buildings and allow duplexes anywhere single-family homes are. Neighborhoods can now put homes and businesses side by side. And cities now have to meet aggressive targets for new home construction.

Of course, the atlas alone won't fix everything. Sometimes outdated, racist, exclusionary zoning rules exist because the people who already own homes have seen their values skyrocket and want to keep things the way they are. It's hard to imagine a map solving that.

"Land-use policy is politics," says Yonah Freemark, a researcher at the Urban Institute who studies housing. "We can say all we want about what the data show, and hopefully that can help make a political point. But in reality, this requires elected officials and community groups to make the case for change. That's a political question, not a data question."

Freemark thinks the National Zoning Atlas is likely to make more of a difference at the state level than the local one. Yes, cities and towns make their own zoning rules. But state governments can overrule them, and the atlas may arm them with the information they need to do so. That was certainly the case in Montana. "Local governments would say to us, confidentially, that they were thankful, because the statewide legislation forces their hand," Cotton says. "They can say it's the legislature's fault: Sorry, NIMBYs, now we're all in on zoning reform."

Still, even when changes are implemented, it will take more than a single legislative session to solve a shortage created by decades of underbuilding. "The state of California has been passing zoning change after zoning change," Freemark says, "but it's still facing low housing construction statewide." But we have to start somewhere — and the atlas offers us a chance, for the first time, to make sense of the housing mess we've created.

It's no coincidence that Bronin, the founder of the atlas, loves Italo Calvino's "Invisible Cities." in the novel, a tale-spinning Marco Polo describes to Kublai Khan dozens of cities in the emperor's far-flung realm that may or may not exist, or may only exist because Polo describes them. That's the thing about zoning: It doesn't map what is (which homes are where) but what could be (which homes are allowed to go where). It's a map of the future — and the future isn't fixed. There's an old saying: The map is not the terrain. That's true. But the right map can change it.

Adam Rogers is a senior correspondent at Business Insider.