Digging deep

Fifty feet below street level, under the garden of an eight-bedroom mansion, what might be London’s most lavish basement is under construction. The equivalent of four storeys, it will house a showroom for a multi-million pound vintage Ferrari collection and a sports hall big enough to fit a tennis court, a swimming pool and a gymnasium.

Jon Hunt, founder of the Foxtons estate agency, is building this underground extravaganza because his home, a trophy property located in Kensington Palace Gardens and protected with Grade II status by UK heritage laws, cannot be expanded in any other way.

Digging down seems like a difficult way to create more living space. Such projects require extensive soil or clay removal, underpinning and waterproofing and can take from six months to two years. But in crowded and wealthy world capitals, where the supply of well-located, quality property is limited, prices seem to be constantly rising and planning laws are strict, homeowners are getting ever more creative. And nowhere is this more apparent than in London.

Hunt, who sold Foxtons for £390m in May, is one of many prominent city residents with grand basement plans. Hedge-fund tycoon Chris Rokos has submitted a proposal to install space for a 16ft-deep swimming pool, complete with high diving board, under his eight-bedroom house in Notting Hill. David Cameron, the leader of the UK’s Conservative party, has had a large basement dug beneath his home in north Kensington. And J.K. Rowling, creator of Harry Potter, installed a swimming pool below the back garden of her five-storey home in Earls Terrace on Kensington High Street.

“There seems to be no limit to what people will do to extend their houses and vast areas of London are being dug up to create [space] underground,” says Jonathan Hewlett, head of estate agency FPD Savills. “People want swimming pools with covers that rise up and over the water to convert the space into a ballroom with a glass floor at the flick of a switch. I’ve also come across underground squash courts, ten-pin bowling alleys and underground parking for numerous cars. It’s more reminiscent of a James Bond film than ordinary life.” (Indeed, one north London homeowner with a below-ground pool gets to it by sliding down a chute that runs from the bedroom down the side of her house.)

These “iceberg” houses, so called because the below-ground additions extend well beyond the property’s street-level footprint, are becoming more common in all types of neighbourhood, as homeowners realise that staying in an expanded version of one’s existing home is often less expensive and less hassle than finding, buying and moving into another, bigger house. In the first nine months of this year, local authorities in Kensington and Chelsea, west London, and Camden, in the north of the city, received a total of 208 applications for planning permission to expand or build basements, up from 140 in the whole of 2004. In Hammersmith and Fulham, further west, requests for such projects have doubled in the past three years, while the number of overall extension proposals climbed by only 60 per cent, according to research from estate agency Knight Frank.

“House prices have gone crazy, so moving into a bigger home in an area you like might not be an option,” says Hugo Headlam, associate director at the John D. Wood estate agency. “If you are happy in the area you live in and the only thing you need is space, why not extend? Your house is probably making more than you do [as its value appreciates] so it makes sense to borrow on that increase and do an extension. Once you consider the costs involved in moving, such as stamp duty, and the services of a removal company, an extension makes a lot of financial sense. A converted cellar becomes an extra floor for a property, allows you to stay in an area that may be close to friends or a good school and it appreciates over the long term.”

Companies that specialise in such projects confirm that business is booming. “Our [turnover] was up by 25-40 per cent last year and looks like it will be up the same amount this year,” says Maggie Smith, sales and marketing manager for the London Basement Company. “Most of that is families who can no longer afford to buy the size of house they want. Digging out the basement is the only thing to do if you want a lot more space. People are digging out under their gardens too now.”

Jason Gabb at Cellarwise agrees. “More and more people are outgrowing their homes,” he says. “We are trying to fit our 21st-century life into a Victorian house and that means we have to work with the space we have to make it accommodate us.”

Smith says that basement extensions tend to cost about £300 per sq ft, making the average job about £200,000. One could move from a £500,000 to a £700,000 house instead but it might require changing neighbourhoods and would certainly involve significant fees – about £28,000 for stamp duty, £3,000 for a lawyer and £4,000 for movers.

That said, elaborate basement projects can be quite expensive too. Her company (one of its completed projects is pictured left) is currently working on a 3,000 sq ft example and the owners are paying £1m just to put the structure in place. One London architect estimates that Hunt will end up spending “tens of millions of pounds”.

The money is needed because digging down is inherently more expensive than building up. Drains need to be redirected and sump pumps installed to avoid a build-up of water pressure. Foundations must be underpinned so diggers can access the space under existing properties. And then there is the problem of making sure daylight, fresh air and, in the winter, heat filters down to the rooms. Anyone embarking on an excavation would typically need to consult a surveyor, solicitor, architect, local authority planning officer, builder, waterproofing company and decorator.

“The most important thing is to work out what you want from the space so you can design [it] accordingly,“ says Richard David, architect and partner at Snell David Architects. “Basements work particularly well when the house is on a slope so it is only partially below ground and the architect can design a window into the main basement room.” But it is not always so easy. “Often [we need] to dig out external light wells into which windows can open. And if you want it as a separate flat you might want to create a separate entrance ... to reduce the claustrophobic feel.”

As for ventilation, builders often need to install extractor fans and “air bricks” at the base of external walls, which have vents through which air can flow. And, since “basements are usually darker and colder than the rest of the building, you may find that a floor heating system works best,” David says.

Michael Hanson, a 38-year-old banker who lives in a six-bedroom property in Fulham, south London, with his wife and two children, recently commissioned a basement (left) that extends from below his house out under the garden, incorporating a series of light wells in the roof. He embarked on the project only after first paying for side and attic extensions; the only place left to go was down. “We needed the extra accommodation to provide the nanny with her own self-contained flat,” he explains. “The other reason we did it is because the gardens are only small in Fulham and we wanted a soft play area for the kids to run around in. The space underneath the house was perfect for this. And the area doubles up as a bar area in the evening.”

The project took more than a year to complete – three months to dig out the hole and then another 10 months of construction. “We were in rented accommodation for a total of 18 months, which was quite a trial for us,” Hanson acknowledges. “But it was well worth it. As well as the enjoyment we have had out of the extra space we also expect to make money on the extension. Houses in Fulham are worth about £1,000 per sq ft but it cost significantly less to put the extension in.” (In fact, the family is now moving out to the country and have put the house on the market for £3.65m through Savills.)

Until recently it was relatively easy to get planning permission for new basement projects since they were seen as less disruptive, both physically and aesthetically, to neighbourhoods than above-ground additions. Local councils tended to require only that the structural stability and amenity of adjoining properties be protected, that party-wall agreements be signed with relevant neighbours and that there be no material loss of open space or important archeological remains. But this laissez-faire attitude could be coming to an end.

Councillors in Kensington and Chelsea have, for example, expressed concerns about excessive subterranean building work in the borough, not only because of the traffic and noise it brings but also because of potential long-term environmental effects, such as increased flooding after heavy rains since the water cannot easily soak away around big underground structures. Prompted by resident complaints about damp in their long-held basements, which they blame on neighbours’ new underground pools and gyms, and worries about subsidence, the council recently commissioned a study of its water table.

“We know it is inevitable that people will want to build down but there is a lot of water flowing underneath Chelsea and we are concerned about the barrier affect,” says Daniel Moylan, deputy leader of the council. “We want to make sure people are doing the research and are not digging down deeper than they should be. If everyone on the street wants to build a three-storey basement then where will the water go? It will have to go down and underneath the extensions. But will it come up again to feed the plants and trees on the other side? Until we know the answers to these questions it seems a sensible precaution to be a bit more cautious on allowing building work to continue.”

Planners are now insisting that each application for underground building work include a full structural engineering survey, which costs about 2.5 per cent of the total project.

In Camden, residents are meanwhile calling for all such developments to be stopped pending a council review of its planning policy. The unrest began in Hampstead, where well-heeled locals have become increasingly angry over the large number of applications for new basements and the disruption caused by the construction.

Sally Doganis, who lives in the South Hill Park neighbourhood of Camden, has endured a neighbour’s basement project for the past year and “there is continual high-pitched drilling, breaking up the soil or clay from under the house”, she says. “There is a noisy conveyor belt that brings out the spoil, mud and earth. And both of these operate all day from 8am in the morning throughout the week, including on Saturday morning,” she says.

Ruth Tamir, a resident of nearby Belsize Park who is trying to block a year-long, 100 sq metre basement project planned by one of her neighbours, is equally concerned about the long-term implications of the work. “Apart from the relatively short-term issues of noise and disturbance it really is about the effect on the water courses,” she says. “I am worried about the impact on the foundations of our house. If Camden council allows this it sets a dangerous precedent.”

Still, it’s unlikely that London’s basement-building frenzy will die down any time soon. Even if houses get larger or the tax on moving house is abolished, such extensions will remain an attractive solution. Where else would people put a swimming pool with a glass room that turns into a ballroom at the flick of a switch? Or a cigar room that doubles up as a cinema? Neither are likely to fit on the roof.