When is a house like an iceberg? When there is more under the ground than above it – a common phenomenon in the houses of the super-rich these days. Never mind using your cellar as a wine store. Now it’s all about digging two, three or even four underground levels to house gyms, swimming pools, media rooms and squash courts – or, in the case of Jon Hunt, the multimillionaire former owner of Foxtons estate agency, your collection of vintage Ferraris.

Basement fever is sweeping the capital. The London Basement Company, for example, carried out 113 jobs in 2006 and has reached 112 this year. Nor is it just the residents of prime central London who are digging down. Subterranean development is all the rage amid the Georgian splendour of Bath, and The Big Basement Company has had calls from Scotland, Manchester and even Portugal.

Simon and Ruth Exelby jumped on the bandwagon early, spending £12,000 on converting the basement of their Georgian home in Shrewsbury into a kitchen in 1999. The work took six months, during which time they had no hot water or cooking facilities, bar a microwave, but the couple and their children, Gemma, 12, and Katie, 10, are pleased with the result. “It’s fantastic,” says Ruth, 43. “We’ve got an Aga now, and it has a beautiful cosy atmosphere.”

Until now, obtaining permission to extend your home downwards has been relatively simple. Planners have been concerned largely with how a property looks from the outside and whether there will be any material loss of open space, as well as ensuring adjoining properties will not be adversely affected.

In June, however, a workman at a property in Wilton Row, Belgravia, died after the basement he was digging out collapsed on top of him. The fatality, combined with growing resentment from residents in wealthy areas, sick of the noise and inconvenience of long-term building works, means authorities across London are beginning to crack down on would-be moles.

Westminster council is looking into tightening the rules on basement conversions, and the Royal Borough of Kensington and Chelsea has commissioned Ove Arup, the design and business con-sultancy, to look into the implications of subterranean development. The borough says its policy is to resist extensions beneath listed buildings.

The Grosvenor Estate, which owns swathes of property across Westminster and Belgravia, has consulted with a QC to see if it can block three basement developments of more than one storey that have been given planning permission. Even in Bath, local agents say it is getting harder to obtain consent.

So, with the planners getting tougher, should you extend your house down a storey or two? And, if so, what do you need to do?

When is it worth it? Digging a basement is probably the most expensive and complicated way to extend your property, so look into going up into the roof, filling in your side return (if your house has one) or extending into the garden (unless you don’t want to lose outside space). Typically, it costs £300 per sq ft to dig out and fully fit and furnish a basement. To extend a side return costs £150-£200 per sq ft, with a loft conversion coming in at about £95 per sq ft.

“Unless you’re in an area where properties are going for at least £500 per sq ft, by the time you’ve had all the aggro of creating a basement, it’s not worth doing,” says Ed Mead, a director at Douglas & Gordon’s Chelsea office. That still covers a lot of areas – at today’s prices, most houses in London, as well as cities such as Oxford and Bath.

You should also take the area into account before rushing to dig. “It needs to be in a street that is well known for having family houses,” Mead says. “If you’re in a street with properties full of couples or families with just one child, then you could be turning your house into something that’s too expensive for the area.”

You should also look at the balance of rooms in your house. “Bunging in a basement is not guaranteed to make you money,” Mead explains. “There’s no point in having one or two bedrooms and three sitting rooms. Don’t try to turn what is obviously a singleton or couple-type house into a family house – it won’t work.”

When doing your calculations, factor in the costs of moving to a bigger house before you discard the idea of creating a subterranean level. With stamp duty at 4% on properties of valued at £500,000 or more, and agents’ fees to pay, you could be looking at a bill of £30,000 on a £500,000 property – which would go a long way towards creating an extension.

“It’s always worth doing if you really want to stay in an area and aren’t prepared to make the trade,” says Grant Aitken, director of Chesterton’s Kensington and Notting Hill office. “Any money you’re going to invest in the basement, you can set against what you would have paid in stamp duty.”

Even if you would rather move than dig down yourself, it might be worth obtaining planning permission. Aitken cites the case of a man who was selling a 1,200 sq ft house in Holland Park, valued at £1.6m. After obtaining consent for a second sub-basement, which would add another 1,000 sq ft, he sold the property for £1.9m – well worth it, even if the process took about 18 months and cost him several thousand pounds. And he didn’t have to endure the digging process himself.

Can I do it? Broadly speaking, you can dig out the basement of any house, although some are more suitable than others. Maggie Smith, sales and marketing manager at The London Basement Company, says older houses are easier to work with because of the way they are constructed – with deep foundations rather than modern concrete piling systems. Things are also easier if you have a suspended timber floor (floor-boards supported by joists), as opposed to a solid concrete one. This means the basement can be excavated from the outside of the property, so you can stay there while the works are carried out. If you have a concrete floor, you will need to dig into it from inside the property.

If you are digging a new basement, or if the exterior of the building will be altered, for example by adding windows or light wells, you will need planning permission. This should be fairly straightforward to obtain unless you live in a conservation area or a listed property.

In the latter case, you may be able to excavate only under the garden, rather than the house, unless there is an existing cavity. Some councils – including Kensington and Chelsea – also stipulate that applications must be accompanied by a full survey of the proposed work by a structural engineer.

Unless you have bedrock under your house (which is harder to cut through), or infill under a garden (which isn’t as supportive), cutting through the ground is not a problem. Flood plains can cause problems, however. Keith Defoe, business manager at the basement development company Cellarwise, says it would be hard to obtain permission for an underground extension anywhere on the London embankment, for example.

You might also require the approval of other authorities, such as the Grosvenor Estate in parts of Westminster and Belgravia. Consider the timing of your application carefully. Grosvenor is trying to stagger basement developments to reduce disturbance, so it may be worth waiting until the people down the road have finished their excavations.

Unless you live in a detached house, you also have the neighbours to consider. At least a month before starting any work, you must issue those living on either side of you with a party-wall notice setting out your plans – giving them the opportunity to object. Be warned: in the event of a dispute, they can appoint a surveyor to protect their interests, and you will have to foot the bill. “Full information is the key to dealing with these awards,” says Simon Price, a party-wall surveyor. “Start the process as early as possible, because it can take several months.”

How long will it take? Quite a while. “From thinking about it to starting to dig, even for a modest basement, you should allow a bare minimum of four to five months,” says Stephen Gray, a director at the architects Weldon Walshe. Obtaining planning permission will take at least eight weeks; drawing up a party-wall agreement should take three, but could require six. For the construction, you’re looking at 8-12 weeks to take the project to decoration level and 22-24 weeks to finish the whole job. “It is noisy and uncomfortable for the first week,” warns Shane Watson, project co-ordinator at The Big Basement Company. Be prepared to move out if you can’t take the strain.

Who should I approach first? First, a lawyer, who will run checks on whether you own the land under your property. Next, a surveyor, to conduct a feasibility survey that will take sewers, electricity and phone lines into account, and make sure there are no serious obstacles. The surveyor will also handle the party-wall agreements. An engineer is critical, to establish what you need in terms of underpinning. It may be necessary to bring in a labourer, to investigate the depth of existing foundations, the level of the water table and the soil type. You will then need an architect to draw up detailed plans to submit to the council. For the work itself, you need underpinning specialists, damp-proofing experts, builders and decorators.

Alternatively, you could hire a company that specialises in basement excavations to do everything for you.

What can I put in my new basement? Anything you like, although they are more suited to some uses than others. “Basement extensions work for functions that need uninterrupted space – swimming pools, media rooms, squash courts,” Gray says. “They’re not so good for overflow living space or bedrooms.”

What else do I need to think about? “The waterproofing details of the basement have to be thought through to the nth degree,” warns Benedict Colgate, group business development director at Holloway White Allom, a high-end building company. Make sure your basement is properly tanked (with a damp-proof membrane that runs under floors and walls to create a watertight space).

The key to turning a basement into a pleasant living space is letting in as much natural light as possible. “Basements can be claustrophobic unless you’ve got natural daylight coming in,” Gray advises. What will help is decent floor-to-ceiling space. “Don’t skimp on the height,” says Phil Spencer, the Sunday Times columnist, presenter of Channel 4’s Location, Location, Location and director of Garrington Home Finders. “It costs more, but it’s worth it.”

Will I need to move out while the work is being done? It depends on your house and how big the project is. If you have concrete floors, there’s no getting round it: you will have to move. And if you’re doing something huge, it’s probably worth being away during the worst bits, to avoid the stress.

Spencer, for example, is having the basement of his house in southwest London excavated, and has decided to move out while the work is being completed. “The rent is extortionate when you’re also paying for a building job, but it’s a lot less stressful,” he says.

How much will it cost? Design fees, which include architects’ drawings, engineers’ fees and planning costs, can range from £10,000-£20,000. Party-wall permissions cost £3,000-£4,000 (plus Vat). Build costs vary enormously, but to underpin a house will cost between £8,000 and £16,000. To dig out a 13ft x 19ft x 9ft basement will cost about £23,000. According to the Royal Institution of Chartered Surveyors, to install a new door, staircase and floor, tank the walls and floors, plaster and decorate would cost about £24,000 – bringing the total to an average of £75,000. Using a specialist company will cost you about £300 per sq ft – roughly the same price as the DIY option, for a similar basement – and this covers design and finishing costs.

Cellarwise; 0808 145 7000, www. cellarwise.com. Holloway White Allom; 020 7499 3962, www.hwaltd.co.uk. The Big Basement Company; 0700 244 2273, www.bigbasement.co.uk. The London Basement Company; www.tblc.co.uk

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