London might once have been an industrial city but now it relies on natural resources. Mining, mostly. At least that’s the impression a visitor would get wandering the city’s most affluent streets. A common sight is the hoarding around a ground floor with a boxed-in conveyor belt sticking out at an awkward upward angle, spouting earth into a skip in the road. This is the sign of the space-mine, the conjuring of domestic square footage from underground as the capital’s prime natural resource.

The issue of mega-basements has been back in the news recently with coverage of the feud between Holland Park neighbours Jimmy Page and Robbie Williams. The one-time Led Zeppelin guitarist is an architecture aficionado who lives in the remarkable Victorian Tower House, designed by the eccentric architect William Burges. He has argued that Mr Williams’ proposed dig would adversely affect the structural integrity of the Grade I-listed house. Mr Williams, who is planning to add to his 46-room house the usual subterranean gym and swimming pool, argues the disruption would be no worse than a passing car. Musos who might once have been throwing TVs out of their windows are now arguing about the finer points of Pre-Raphaelite interiors.

The basement boom started not in prime postcodes but among the merely-prosperous middle classes of Fulham and Wandsworth, with families expanding their cellars to accommodate relatively modest playrooms and dens. But both developers and the super-rich spotted an opportunity. Big basements had always been too expensive to be worthwhile, until London’s increasingly insane property prices flipped everything.