For London's multi-millionaires not even tough planning rules which restrict buildings from expanding upwards can stop owners increasing their square-footage.
More and more of the capital's super-rich are expanding downwards instead, and digging out mega basements.
But there's a problem. The diggers go so deep below ground, it can be difficult to get them out again. So what happens?
This week's New Statesman has a rather surprising claim - that they are simply buried in the ground. Dozens of diggers are just left there, predominantly in Belgravia, Chelsea, Mayfair and Notting Hill. Writing in the New Statesman, Ed Smith says:
So how many diggers might be down there? No one knows for sure. One developer estimated at least 1,000.
However, speaking to the Evening Standard, Kevin O’Connor, managing director of Cranbrook Basements, dismissed the claim.