Room at the bottom: Creating or converting a cellar

Sam Dunn goes underground to look at the costs, benefits and pitfalls of building a new living area below stairs
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Ask homeowners to describe their cellar or basement and many would talk unlovingly of a dank, musty storage space packed with boxes, bicycles and tools.

Others, including the Tatman family, might beg to differ. Carolyn and Andy are putting the finishing touches to a new, warm and brightly lit living space in their Birmingham home where they will relax, watch television and listen to music. "We now have an extra floor," says Carolyn. "It's phenomenal. We can't believe it's part of our house."

It took three months, cost £29,000 (a sum drawn from savings and a remortgage) and required some excavation work to make the room bigger, and add waterproofing and windows.

"I'm sure it will add value but we'd always thought about how we needed an extra room for our children to use."

The trend for converting cellars or even excavating to create a new basement floor is gathering momentum in the UK, particularly in cities. Soaring house prices regularly force urban dwellers to abandon plans to move to a bigger home, and planning restrictions curb expansion upwards or outwards, says Alan Tovey of the Basement Information Centre, which helps homeowners consider whether to "go underground".

Digging down to create a playroom, home office or gym will add a whole new layer to your home and can increase its appeal to future buyers. But depending on where you live, one big obstacle remains: price.

The cost of a new floor in a central London home can nudge £100,000, says Mr Tovey. By contrast, you could expect to pay an average of between £20,000 to £30,000 outside the capital, adds Andy Parkes of Midland Tanking, a firm that provides waterproofing for basements.

But the decision whether to create or convert a basement or whether to move to a bigger home shouldn't be dictated solely by where you live. Much will depend, too, on the size of the property and on your own specifications.

"Where the cost of moving for more space is very high - in London, for example - a £100,000 conversion might well be worth it, since it's economically viable," says Mr Parkes.

The high cost of the project is down to the nature of going underground. A lot of work will be required to make the new area accessible, and the foundations may need to be reinforced and new underpinning introduced. There must be waterproofing, insulation and a source of light.

"Compared to a loft, it's a major undertaking, so don't underestimate it. You need to know what you want and stick to your guns with contractors," says Roy Ilott, spokesman for the Royal Institution of Chartered Surveyors.

During the process, you can expect to consult or hire a surveyor, solicitor, architect, local authority planning officer, builder, waterproofing firm and decorator (see the panel below).

While you may enjoy adding a whole new dimension to your house, don't forget that others may be less enthusiastic. Home improvement research from the Woolwich shows that a basement won't always boost your home's value. "Done badly, it can hit the selling price," says spokeswoman Ellie Waters.

"Unless it's in a particularly desirable place where space is at a premium, you shouldn't expect to get your money back straight away."

To increase the value of your property, the Woolwich research suggests an extension (the average cost, nationwide, is £20,000 for a single storey) or loft conversion (average cost £15,000) instead.

One of the cheapest ways to "buy" a basement is via a "self-build" house you design yourself, says Mr Tovey at the Basement Information Centre. This allows builders to start construction with the cellar. "It's usually a third of the cost of a 'retro-fit', or putting a basement in afterwards," Mr Tovey explains.

A growing number of UK housebuilders are now considering making use of basements to create extra space in new developments, says Mr Tovey. While nearly every new home in Germany has a cellar, building firms in the UK have until now tended to shy away from the expense and trouble of digging deep into the ground.

As with any major home improvement, you should consider different ways of paying for your new living space before embarking on the project. Instead of taking out a personal loans or using savings, look at a remortgage if your home has risen in value and you can afford to release some cash.

Construction plans can easily change, so when working out the cost, expect to go over-budget and factor in another 15 per cent to be on the safe side.

www.basements.org.uk

HOW TO GO BELOW AND KEEP YOUR HEAD ABOVE WATER

Expect upheaval

To turn a leaky cavern into a watertight living area, plan for at least two months of mess involving excavation, the underpinning of foundations and waterproofing.

Much depends on soil type and the water table.

Plan ahead

You will probably need planning and regulatory approval from your local authority. "As a rule, consider your planning authority if you're going to change something," says Roy Ilott at the Royal Institution of Chartered Surveyors.

The law

You shouldn't have any legal problems digging beneath your house, adds Mr Ilott, but check with the relevant authorities for water and gas pipes, sewers and, in London, Tube lines.

Structural stand-off

If extending or excavating beneath the "party wall" - a boundary wall shared by you and next door - keep neighbours fully informed.

Space

There is no statutory headroom rule, but 2.3 metres is regularly advised for comfort.

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