Excavating under your house can add valuable space

So you have converted the attic into a children's bedroom and added a conservatory at the back of the house. No more expansion is possible - or is it?

You could dig out a basement under the house. It sounds expensive and messy, but spiralling house prices mean that excavating a basement can add considerable value.

According to Eric Morson, of Construction Going Underground, a specialist in subterranean building, adding a basement to an average house will cost around £25 a square metre to build. With inner London house prices now averaging about £50 a square metre, it makes economic sense.

Basements fit in with modern lifestyles too, Morson says. A vast projection television and ear-splitting audio system is ideal for a basement because windows are not necessary and it is much easier to keep the sound in.

Basements often make sense for buy-to-let landlords and owner-occupiers who want to make some money by letting part of their house. Unlike loft conversions, basements can have their own front door, accessed by stairs down from the front garden.

Morson has just opened what he claims is the world's first "show basement" in north London to demonstrate what can be done. The firm's offices had a dark, dank cellar that was excavated to provide decent headroom, underpinned, and provided with skylights for natural light.

The basement has been fitted out with a kitchen, bathroom and the latest craze for the rich but nervous - a "panic room" with an assault-proof door, inside lock and a direct line to summon help.

Getting a basement installed sounds like a nightmare but most families need not even move out while the work is done. The contractor will usually dig out the front garden and excavate under the house from there.

The essential thing is not to spoil the job by economising. "When we started, people only did the basics, but it is pointless spending that sort of money unless you make the basement an integral part of the house with the same feel," he advises. "That way you get genuine added value."

Philip and Hilary Solt had a basement excavated under their house in Westover Road, Wandsworth, four years ago to provide Hilary, an interior decorator, with an office.

"Originally it was just going to be my office down there but we wanted daylight back and front so we had the whole area under the house dug out," she says. "Now we have a bedroom, shower room and utility room as well - almost a two-bedroom flat."

Specialist contractors London Basement Company worked under the house while the family tried to stay calm above. "It was scary but we stayed in the house," Hilary says. "They dug out the front garden first, put a barrier round it and extracted everything through the front."

"It provided a huge amount of living space while the children were growing up," Hilary says. "It is a room for all seasons, so cool in the hot summers we have been having recently, and in winter the underfloor heating makes it the cosiest room in the house."

The process took six months and cost £220,000. Hilary Solt does not believe the basement adds much to the actual value of the house, but this is now under the ultimate test - the Solts are moving out of London and the house is on the market with estate agent Carolyn Winbow of John D Wood.

"Yes, the basement does add value, definitely," she says. "We have got a similar house close by without a basement on offer at £925,000, and the Solts house is on the market at £1.35m."

If the house sells at that price, it values the basement at more than £400,000 - a substantial return on the investment.

Buy-to-let investors often find a basement flat lets well, says Ruth Barr, lettings manager at Knight Frank's Wimbledon office. But the conversion must be carefully planned.

"The garden is often an issue," she says. "You either lose the garden or try to rent it out without any outside space. If you let the tenant have the garden, they can feel very overlooked.

"And it is better to create one big bedroom rather than two poky little ones."