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."