Subsidence predominantly affects those properties on clay soils, which includes many areas of London and across the country south of a line between Hull and Bristol. Dry weather causes soil shrinkage, which creates movement in building foundations, resulting in cracking. Cracking can also occur when dry soil swells because of increased moisture or as a result of mature trees being removed.
Figures released last week by the Association of British Insurers show that subsidence claims in the first quarter of this year were double those of last year. This seems to confirm the predictions of the Chartered Institute of Loss Adjusters that this summer will see at least a 50 per cent increase in claims.
Tony Boobier, a director of Robins Response, a firm of loss adjusters, says: "We are already recruiting more adjusters, surveyors and engineers to meet the coming crisis. The warning signals are coming loud and clear from the water industry: 60 per cent of the country is going to suffer a drought and 15 per cent a severe drought." The measures being taken by the water companies to replenish their supplies are damaging some properties. As water is taken for the first time from some rivers, so the ground under nearby properties will dry out and crack.
Surveyors preparing homebuyers' reports are now more likely to call in structural engineers after cases in which they have been sued for failing to spot warning signs of subsidence.
Meanwhile, homeowners suffering subsidence may be left with a property that costs more to insure, cannot be sold without a heavy price cut, and cannot viably be remortgaged. Alison Collett, a journalist who lives in Wimbledon, has a 2-millimetre crack running across her living room which was caused, according to a structural engineer's report, by a combination of dry weather and 10-metre roots from a rose bush. "It is frustrating to think of these lower interest rates around, and think that the building societies won't touch my property," she says.
In fact lenders will consider a property afflicted by subsidence for remortgaging, but they are likely to require further surveys at the homeowner's expense. In many cases that will eliminate any financial gain from switching mortgages.
If predictions of a dry summer are accurate, insurers stand to make big losses. If the rise in the first quarter's claims is maintained, the total bill will be pounds 650m - the highest since 1990, one of the worst years on record. As a result homeowners can expect to see big increases in premiums. However, it is unlikely that premiums will increase across the board, as insurers are using sophisticated databases and computer maps that should accurately assess the subsidence risk for each property.
It is also important for homeowners not to become locked into a victim syndrome. Effective action will reduce the risk and some insurers are talking of limiting payouts where owners have contributed to the problem. A free booklet published by Direct Line*, available to non-customers, could help to pinpoint problems. It points out that while three in four claims involve clay, subsidence is also caused by split drains, so any leaks should be repaired quickly.
The most common contributory factor in subsidence remains trees and shrubs. A rule of thumb is that no tree should be planted closer to a home than its mature height, though it may well pay to consult a tree surgeon before removing trees or shrubs.
q Contacts: Subsidence Claims Advisory Bureau, 01424 733727; Arbicultural Association, for a list of tree surgeons, 01794 368717; *Direct Line, 01473 824447.