It is the stuff of nightmares. The hairline crack you thought was just an acceptable sign of ageing plaster gradually widens. Then the brickwork follows suit. For many homeowners, panic is the next step. But, as Robert Liebman discovers, for a small band of would-be buyers subsidence is an opportunity, not a problem.

The house had visible subsidence. It had once been divided into two self-contained flats which would have to be restored to a one-family home. And the seller was uppity: exchange in 10 days or the deal was off.

Eva and Peter Botterill, experienced homebuyers who, with two grown children still in the nest, needed a large property, had no objection to living in Kew Gardens, in London. Most importantly, they needed a bargain and are fearless.

"Roots from a nearby tree had cracked the drain, causing a wall to collapse," Mrs Botterill explains. "An interior wall had a huge crack running through it. But this was a solid 1930s houses. I knew it wasn't going to fall down. And the subsidence had kept the price low."

As work on the Botterill home progresses, a handsome, spacious and sturdy home is emerging. Nevertheless, "subsidence monitoring" as a mortgage condition should serve as a warning to those with tender nerves or thin wallets.

Houses afflicted with subsidence may be heavily discounted. Some buyers in a market starved of quality properties may regard such damaged goods as better than nothing. Actual or would-be speculators will be attracted to the prospect of big profits when they sell on, while estate agents may well have hard-to-sell properties of that sort on their books. But risks abound, often hidden in complex structural and insurance issues. Professional information and advice are essential.

Despite the risks, the Botterills had few doubts about the wisdom of their purchase. Originally a three-bedroom semi-detached family home, the property had been divided for two elderly sisters. Subsequently inherited by an Australian relative, the house was then bought by a local estate agent. "When we viewed it, the seller told us it was already under offer, and we offered pounds 5,000 more. He accepted, provided we exchanged contracts in 10 days," Mrs Botterill says.

"But we were worried about another buyer moving even quicker. The seller had accepted a higher offer from us, so he could accept even more from someone else. He told us other people were interested. I'd have been silly, in a rising market, not to believe him. You can't be complacent if you want something.

"The seller's deadline did not give us much time for surveys so we got a surveyor friend of ours to look at the property. He identified the tree root as the culprit but reassured us that underpinning would not be required." A wall-strengthening procedure known as brick-stitching would suffice.

The couple paid a survey fee of sorts. "He advised us that if he put it in writing and charged us, the building society would take it seriously," Mrs Botterill adds. "If he had said, `Don't touch this with a bargepole', his advice would have been free."

And if they had not had a surveyor for a friend? "If you wait for a survey or pick a slow solicitor, you won't get the property," she insists. "You can't risk going through proper channels in a rising market. If necessary, I would have hired a local surveyor to provide an on-the-spot survey. Surveyors don't want to lose a fee."

Insurance was the next hurdle. "We had to make sure that the previous owner's insurer accepted the claim, so we had to get the benefit of the claim assigned to us."

The Botterills had been living, unhappily, in a rented flat after selling their home in Chiswick. Cash buyers, they nevertheless needed a mortgage for rebuilding work estimated at pounds 50,000-60,000.

Mrs Botterill explains that the building society cooperated. But, she adds: "They put a full retention on the property to ensure that we reinstated it to a single dwelling - one kitchen, one set of meters. There had to be subsidence monitoring and professional rebuilding."

Ten days is more than adequate for a proper survey involving a drains test and soil sampling, according to Malcolm Hollis, a chartered surveyor. Soil information on specific areas is available from computerised data bases, "and we can also do an auger test at the site to take a plug out of the ground which provides a sample down to perhaps three metres. But surveyors do not have x-ray vision. There is always a level of risk".

Mr Hollis notes: "The downside of simple brick-stitching is that if the foundation is not replaced, and if movement continues because the procedure tackled the symptoms and not the disease, future repairs might be extremely costly and not covered by insurance."

Friendships may also be sorely tested. "Even surveyors can get it wrong," warns Richard Berns, senior partner with a London firm of solicitors, Piper Smith & Basham. "Normally, buyers should check that the surveyor has proper negligence cover for an adequate amount. Preferably, your surveyor sits on the panel of the building society, otherwise the survey might not be accepted."

Mr Berns adds: "Buyers must appreciate that the fact of a subsidence claim has to be disclosed to future purchasers. A history of subsidence will put some potential purchasers off, particularly in a depressed market."

Subsidence usually involves the kind of work that means the home must be vacated. The Botterills cheerfully moved into a building site. Mrs Botterill points to the room that will get new French doors and to the rubble-strewn section of the garden where her kitchen will sprout.

She expects to live in tip-like surroundings for three months, followed by three more months of substantial decorating and interior work. "Then we will think about things like whether we want a pond in the garden. We won't be finished for several years yet," Eva says.

The omnipresent skip and the mud and dust are not entirely unwelcome. "We lived in our previous house for 15 years. Peter didn't have the enthusiasm to do it up again, and I was also ready for a change. If you don't change, you stagnate. We now have a project we can actively share for the next five or six years. It's good for us. It's good for our marriage."

For advice on subsidence: Royal Institution for Chartered Surveyors, 12 Great George Street, Parliament Square, London, SW1P 3AD, 0171 222 7000; Association of British Insurers, 51 Gresham Street, London, EC2V 7HQ, 0171-600 3333; Piper Smith & Basham, 31 Warwick Square, London, SW1V 2AF; 0171 828 8685; Malcolm Hollis, 01700-127000