Over the last half-century in America, it's become acceptable, then increasingly common, then entirely unremarkable, to live alone. Women who once lived with their families until their wedding day now live alone. Men delaying marriage later into their 20s live alone. Divorcés, more common today than in 1950, live alone. And seniors who live longer now than ever before — and who are less likely to spend those years in a retirement home — increasingly live alone, too.

As a result of all these shifts, more than a quarter of households in the U.S. now contain one person, alone. In 1940, it was about 7 percent:

This trend has all kinds of consequences, including a particularly problematic one for where we live: Our housing stock wasn't built for a society full of singles. Our communities instead are full of homes meant for the traditional nuclear family — two-bedroom starter homes, three-bedroom houses, apartments with more bathrooms than a singleton needs, full-service kitchens when 25-year-old bachelors now primarily dine by microwave.

We're increasingly a nation of single people, but we're still living, quite literally, in a world built for families.

The disconnect between these two trends is particularly acute in big cities, as the Furman Center points out in a new research brief. In New York, Austin and Denver, nearly 57 percent of adults were single in 2010 (although not necessarily living alone). In Washington, D.C., that figure is a whopping 71 percent.

But none of these cities have anywhere near enough small-sized housing to accommodate them. That means that a lot of people are probably living with unrelated adult roommates who'd prefer to live alone (half you people in D.C. group homes?). And it means that some people who do live alone are likely paying more for space they don't want in a large one-bedroom because there aren't enough alternatives in studios and efficiencies.

Changes in demographics and social norms invariably occur faster than changes in the built world around us. It took decades, even more, for Washington and New York to grow up into the places they are today. But, as Vicki Been, Benjamin Gross and John Infranca at the Furman Center point out, a lot of cities are also actively making it hard for the housing supply to adjust.

The rise of singles calls in particular for more micro housing: apartments the size of studios or even smaller, and "accessory dwelling units" (think in-law cottages or garage apartments) that might be built in the back yard of existing homes. It also calls for a different model of housing where, for instance, four singles might share a communal living space adjacent to their separate units instead of each having their own living room.

Neighborhood opposition and existing regulation make this kind of housing hard to build in most cities, though. Parking requirements, for example, often mandate that new housing come with new off-street parking spots, too. But that rule is impractical for someone who wants to rent a cottage in her backyard. And it makes projects financially unworkable for a developer who wants to build an apartment full of micro units next to a train stop for residents who don't own cars.

Other laws set minimum standards for how small a housing unit can be — in much of New York, it's 400 square feet — making micro units effectively illegal. Or they limit density by capping the number of units that can be built on a given plot of land. Then there are laws that tell property owners how much of their land they can build on.

Cities would have to change many of these regulations to make more micro housing possible, and there are a few places where that's starting to happen. Washington is in the process of revising its zoning code, and some changes could potentially enable more accessory dwelling units. New York has begun to test the idea of micro housing units with the development of a 55-unit modular apartment on city-owned land in Manhattan that would otherwise violate city code.

Many of these smaller units are decidedly up-scale, with high rents and communal luxuries like club rooms. But micro housing could also, in theory, be affordable housing, making cities like Washington accessible to single people who can't even afford a studio today.