The revised National Planning Policy Framework (NPPF), published in July, eases the rules on upward extensions. Councils will be empowered to support “opportunities to use the airspace above existing residential and commercial premises for new homes”.

Building up, instead of down or sideways, is a welcome step-change in helping to alleviate London’s housing shortage.

With land scarce in the capital and the population growing, the problem of affordable accommodation is getting worse. As a result, the mayor of London’s London Plan is targeting 65,000 additional homes a year.

From green to brown to sky blue

Despite the chronic need for affordable homes, people are strongly against building on the green belt – and the government agrees. In the NPPF it reiterates its commitment, yet the London metropolitan green belt is under enormous pressure.

Building on the green belt is only supposed to happen under “exceptional circumstances”.

Prioritising building on brownfield land and high-density schemes are put forward as solutions, but neither are easy. Looking up is the practical answer. Airspace development has huge potential to create affordable homes for people, as well as reduce pressure on the green belt.

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It works like this: developers buy air rights from freeholders who turn the intangible, the airspace on top of their properties, into something tangible – profit for themselves and new homes for Londoners.

Our research at Apex Airspace has identified that 360,000 homes, equivalent to 720 acres of developable land, could be built on London’s rooftops – a cost-effective, minimally disruptive use of space. And 120,000 homes could be built above council blocks and housing associations, addressing the shortage of affordable homes for people on lower incomes.

A note about quality

Not all roofs suit airspace. The roof needs to be surveyed to see if it can take the additional weight safely. It’s best if it’s flat and the building is low rise, so the airspace doesn’t detract from London’s skyline. This meets NPPF rules that rooftop developments must blend in with the neighbourhood and be well designed.

The quality of modular housing is easier to control than traditional construction on land. Prefabricated new homes by Apex Airspace, for example, can be built in a factory’s quality-controlled environment, then installed on a roof by crane over a day or two. This reduces the impact on the neighbours below and minimises disruption to London’s under-strain road network.

In the next 12 months, Apex Airspace is building 130 new homes and has signed partnerships with housing associations in London boroughs and big businesses.

It’s a promising start to using untapped, unused space to address the capital’s housing shortages, help the mayor of London meet housebuilding targets and alleviate pressure on the green belt.