When people talk about cities seeming futuristic, generally they mean it in a positive way. But Hong Kong — a wealthy metropolis plagued with an affordable housing crisis — shows what could happen as cities get more crowded.

According to United Nations forecasts, cities across the globe are poised for explosive growth over the next few decades. Some 2.5 billion people are expected to move into urban areas by 2050, the UN estimates.

These predictions have urban planners scrambling to figure out where all those people will live. Some envision high-tech and modular living spaces. Others predict the trend of coworking will morph into coliving, and people will come to terms with shacking up with strangers.

But Hong Kong offers a different vision for the future. With only 7% of the island's and is zoned for housing, Hong Kong's chief executive, Leung Chun-ying, has called the housing crisis "the gravest potential hazard" to society.

Bed bugs, bad smells, no space

Scattered around the island are microapartments filled with so-called "coffin homes" — rooms of 400 square feet subdivided into 20 or so units, which many inhabitants refer to as "cages." Some are actual wire boxes.

The spaces are just big enough for one person of a medium-build to lie down. If they need to use the bathroom or cook dinner, they must share a communal space with at least a dozen other people. Families are routinely separated due to the lack of space. Smells are putrid, and there are often bed bug infestations, the South China Morning Post reports.

A woman sits in her 25-square-foot "cubicle," or "coffin" home. Bobby Yip/Reuters Roughly 200,000 people around Hong Kong have been driven to live in these coffin homes — largely because there are few affordable alternatives for recent college graduates and the elderly. The starting rents are about $180 a month. The only amenity is the location, as many residents live just steps from the bustling shopping and financial district.

"In Hong Kong, everyone's apartment is horizontal — you don't really experience a vertical spatial quality within (people's) houses," architect Lap Chi Kwong told CNN.

How to move forward

The good news is that urban planners have already envisioned a more humane future, turning cramped apartment buildings into more livable spaces. One design is "Towers Within a Tower." It's a multi-floor set of rooms that go vertical rather than expand outward. A single-occupancy tower measures between 350 and 450 square feet.

Kwong, who lived in Hong Kong for 15 years, is one of the designers on the project. "We wanted to provide public space that people can enjoy — where they can communicate and play around each other," he told CNN.

The biggest trade-off in accommodating millions more people is deciding what certain spaces ought to be for. In Hong Kong's case it could mean increasing the proportion of land zoned for housing, up from the current slice of 7%. In New York City, one of the largest urban areas in the world, 75% of land is zoned for residential use.

If cities don't start building smarter, cases like Hong Kong could become less of the exception and more of the rule.

