Cities are increasingly aware that doing nothing is not an option.

“There’s no perfect answer, but we are beginning to see the emergence of some ideas that have the best chance of arresting the worst of the impact of rising city populations,” says Challis.

In London, the answer so far has simply been to build more. The UK capital’s mayor, Sadiq Khan, has targeted the construction of 116,000 affordable homes by 2022. In the UK financial year to the end of March this year, work had begun on 14,544 affordable homes.

“Solutions can focus on expanding development and sharing the cost of new supply,” says Challis. “But that alone won’t be enough.”

Homes are already getting smaller – not quite to levels seen in Hong Kong – but they’re getting closer to the 37 sqm legal minimum size of a new one person, one bed flat in London. New methods, such as modular construction, can help to reduce the time and cost required to build more affordable homes in one of the world’s most expensive cities.

Japan’s biggest housebuilder, Sekisui House, recently ploughed £22 million of new equity into a modular homebuilding venture in partnership with the UK government’s Home Building Fund.

“Shared-risk solutions in funding development, combined with innovations in planning, are a step forward,” says Challis. “As is the arrival of more professional, institutional capital able to deliver the homes that cities such as London so desperately need.”