As cities around the world consider how to house their rapidly growing populations, modern building methods are helping them to make better use of underutilized airspace above existing transport infrastructure.

Across the world, over four billion people live in cities – with that number projected to rise by 54 percent by 2050, when around 68 percent of the global population will live in urban areas.

In cities like London, housing is already in short supply, primarily due to a lack of land available for development. Yet according to a study from engineering firm WSP, a significant amount of space over the city’s open-air railway lines is underutilized, and could be used to build more than 280,000 homes.

“While not a silver bullet, building in cities’ airspace above transport infrastructure presents a promising way to ease some of the pressure for more urban housing,” says Nick Whitten, JLL’s UK director of residential research.

In London, a development at Tower Bridge is already underway, with plans to build over active train lines around stations at Clapham Junction, Victoria, and Willesden Junction in the pipeline.

In Australia, attention is turning to the space above Sydney and Melbourne’s train tracks to build high-rise housing developments, parks, and sports stadiums. And in New York, the ambitious Hudson Yards project will see an entire 26-acre neighborhood, complete with 17 million square feet of buildings, suspended over 30 active train tracks.

“New modular building technologies, which make it possible to build above railway tracks while ensuring the safety of the workmen above and trains passing below are facilitating these developments,” explains Whitten. “Such methods, which are becoming more prevalent in the UK, allow for high-quality residential buildings that can also be constructed more quickly than traditionally-built high-rise towers.

Changing perceptions

One of the key challenges for developers, however, is the public perception of housing built directly over train tracks. “People might assume these developments are not desirable,” says Whitten. “To make them desirable, aspects like tech and design solutions must be better than ever. For example, products that allow the buyer or renter a level of control over the design of interiors could make them easier to market.”

In addition, Whitten believes that the centrality of railway stations in big cities – and if nothing else, the prospect of an easier commute – plays a key role in garnering buy-in.

“If the location is good, the rest will fall into place in growing cities,” he says. “Plus, as pioneering developments over rail lines become more common, people will become more open to the idea.” In Chongqing, a city in south west China, a metro line even runs through the middle of a residential tower.

Making more of car parks

Open air car parks are another underutilized development opportunity in central urban areas. A JLL study shows that around 80 percent of urban car parks across the UK are at surface level, and could have developments built over the top while maintaining the car park space at the bottom. Such developments could create an estimated 400,000 homes across the country, with 75,000 of those in London.

“There are so many spaces in cities that can be turned from single- to multi-use to unlock more value from the land,” says Whitten. “Urban living is becoming more dense and city authorities need to think creatively about layering the usage of space and maximizing its potential. We’ll see new developments and regeneration projects increasingly reflecting that.”

As more cities come to realise the value of their underused airspace and how it can help them meet today’s and tomorrow’s urban challenges, it could get crowded in the skies in years to come, Whitten believes.

“Not only will we use airspace to help solve the housing crisis and fulfil locational aspirations, but in 10 to 20 years we may start to see competing use from drones and perhaps other flying utility vehicles,” he says.

And this in turn could have a big impact on how buyers view high-rise properties. “If urban traffic does take to the skies, lower floors could eventually become more desirable than upper storeys,” he concludes.