Digging up the garden used to mean preparing the vegetable plot or weeding flowerbeds. But today, it often refers to those embracing outsized subterranean extension projects in ‘cash-rich, space-poor’ urban areas. Extravagant examples of the current craze for going underground in the ‘City bonus belt’ of Kensington, Notting Hill, Holland Park and Knightsbridge abound. Typically, basement projects range from installing squash courts, garages and gyms. Then there is the double-height room with the space for the owner to spring off a high diving board into a pool. But most of these are trumped by the west London home of a banker that now has a 30ft abyss that will house a 52ft swimming pool, spa with a Jacuzzi and treatment room, vast kitchen and parking for six vehicles.

It is believed work on the £12 million house will cost about £2 million. As absurd as this low-down phenomenon might sound to country dwellers, every square foot counts in Britain’s cramped towns and cities. As planners and conservation bodies insist on preserving the footprint of houses within certain para-meters, an ambitious basement extension is a good way to create extra space, argues Trevor Abrahamsohn, managing director of Glentree (020–8458 7311; www.glentree.co.uk) in north-west London. And once you reinstall the garden on top of the domestic excavation, no one will even know anything has happened below, he says.

Mega-basement projects, the archaeological digs of the property world, are not for the fainthearted or tight-fisted, however. You need to pay from about £300 a square foot and employ a battery of expensive specialists a solicitor, surveyor, engineer, damp-proofing expert and builder to carry out the work. However, once you take into account removal costs, estate agent and solicitors’ fees and Stamp Duty (totalling about 7% of a house’s value), it could make sense to build a sub-basement rather than move to a bigger house. ‘Rooms that don’t need windows, such as a gym, Turkish bath, wine cellar, media room and pool, are best suited to being hidden under the surface of the earth,’ Mr Abrahamsohn suggests, ‘although you can do wonders with light wells that admit natural light.’

Silverwood on The Bishops Avenue, for sale with Glentree at £16.5 million, is a good example of how light pours into rooms that require it, with staff quarters, a pool, cinema room and study on the lower ground level. Late last year, developer Landmass Properties (www.landmass.co.uk) dug out the lower quarters of a mews house in Grosvenor Crescent Mews, Belgravia to create a media room, Zen garden, laundry room, bar, library, steam room and gym/study, with a dramatic nine-metre waterfall cascading down from the first storey to the Zen garden. Luxury bunker space doesn’t come cheap and the expanded house is now on the market (price on application) through Knight Frank (020–7591 8600).

Alan Waxman from Landmass Properties believes ceilings need to be at least 10ft high and good design is crucial, too. ‘You need to think about where you should excavate to get your money back or make a profit when re-selling. What you do in Knightsbridge or Belgravia might not make sense in Balham,’ he points out. Digging deep is not only a London pursuit, finds Jeremy Fisher from Jeremy Fisher Building (020–7731 0716; www.jeremyfisher.net), who renovates houses and extends downwards. He is getting enquiries from owners of listed period houses in Hampshire and
Surrey, who want to make their homes comfortable for the modern family.

‘Typically, the rooms are quite small, and the listing means you can’t touch architraves or even doorknobs, which makes refurbishment difficult. By digging down, you can produce large contemporary spaces a music room, playroom and large family kitchen that look as if they’ve been there forever.’ Some authorities, including the Royal Borough of Kensington and Chelsea and the city of Bath, are becoming less enthusiastic about giving consent for going down and out, cautions house finder Richard Leatham from The Buying Solution (www.thebuyingsolution.co.uk). ‘There are fears of what this might be doing to the sub-structure. Get a proper report confirming that the roots of neighbouring trees won’t be an issue, and beware of underground rivers.’