Age: As old as the world's first wine cellar.

Appearance: Just like any other mansion.

What are iceberg homes? Homes like icebergs.

Freezing cold? No.

Prone to sinking cruise liners? Not quite.

Melting? Not really. It's called an iceberg home because, as with an iceberg, quite a lot more of it is lurking beneath the surface.

And where are these iceberg homes lurking? All over – and, crucially, under – London's Royal Borough of Kensington and Chelsea. The city's super-rich have been digging down beneath their townhouses to make space for subterranean swimming pools, gyms, cinemas, ballrooms and servants' quarters, much to the consternation of their neighbours.

Why are the neighbours consternated? Partly because of the noise and disruption of up to two years of building works.

And? And partly because in a few cases the digging has caused their own homes to fall apart.

Really? Yup. Excavation work under Goldman Sachs director Christoph Stanger's mansion has caused shifts in the foundations, forming cracks and trapping neighbouring residents in their flats behind doors that no longer open.

And what are the council doing about it? Until recently, almost nothing.

Seriously? Seriously. Of around 1,000 planning applications for basement extensions made in the last five years, more than 800 have been accepted and – as of November – only 90 refused. Planning laws prohibited building upwards, but, as one architect put it: "There was nothing to stop us from drilling all the way down to the south pole."

And did they? Some are getting close. Canadian TV tycoon David Graham, for example, has produced plans for a four-storey basement bigger than his house itself.

But now the council's going to put a stop to it? Not really. New draft rules will prevent digging under listed buildings and reduce the amount of a garden that can be excavated, but they don't come into force until the end of the year.

Meaning? Meaning for now they may just cause a rush on planning application forms and massive drills.

Don't say: "Well, the servants need to live somewhere."

Do say: "Why not go to an actual bloody cinema?"