Properties at risk are those built on shrinkable clay soil, most of which lies south of a line between Hull and Exeter. In normal weather, houses move slightly as the soil dries out towards the end of the summer and swells again with winter rain. But two unusually dry years have sent the cost of insurance claims for subsidence damage soaring, from pounds 125m in 1994 to pounds 331m in 1996. This year's combination of dry weather plus more properties being surveyed in an active market is likely to result in even higher claims.
But while home-owners lie awake at night listening for every creak and fearing the worst, experts are reassuring. "Almost any building on clay soil moves up and down between summer and winter," says Mike Crilly, project manager at the Building Research Establishment. "People panic when they see a crack, which is understandable, but almost all buildings crack for some reason during their lifetime. Subsidence damage shouldn't be that serious. Your house is not going to fall down. Generally its performance is hardly impaired and the damage is purely aesthetic."
Properties most susceptible are neither the oldest nor the newest, but those built between the Forties and about 1970. "Building materials were more inflexible than before, but foundations were not up to today's standards." Anyone buying a Victorian or Edwardian house can expect to live with a few cracks, many of which will appear in summer and disappear in winter.
The BRE has drawn up a classification of subsidence damage with recommendations for action. These are set out in a useful layman's guide, Has Your House Got Cracks? by TJ Freeman, GS Littlejohn and RMC Driscoll (published with The Institution of Civil Engineers, price pounds 9.95.) Category 0 means hairline cracks causing cosmetic damage only. If you are unfortunate enough to have cracks wider than 25mm, or the walls lean badly, the windows break and roof timbers lose their bearing, your house is in category 5 and may even fall down.
Category 2 signals the time to take action. This is when you have several cracks at least 3mm wide, possibly on the inside only, that do not close up in winter. The solution for this fairly minor damage could be as simple as pruning or removing thirsty trees. Trees are at least partly responsible for about 80 per cent of all clay soil subsidence claims. No tree should be planted nearer than 5m to a house, and willows should be at least 40m away. But pause before you reach for the axe. Removing trees may cause more problems than it solves.
"Properties on clay soils can suffer from opposite and equal perils," says Mr Crilly. "You may have heave when you remove a tree because the ground swells as it takes up water. In general, it is fine to remove trees if they are not older than the property, because the house simply goes back to where it was before the tree was planted. If the tree is older than the house, seek specialist advice."
Underpinning is very much the last resort these days. "Householders do not appreciate how much of a disruption and how expensive underpinning is ... and insurers are reluctant to sanction it for most properties." The bill for underpinning a semi could run to pounds 20,000. Even if you are covered by insurance, you will probably have to pay the first pounds 1,500, and your house may be uninhabitable while the work is being carried out.
Home-owners in subsidence high-risk areas may also have to pay higher premiums, and properties with a history of subsidence can be hard to insure. "There is a very real subsidence blight once properties have had to be underpinned," says Chris Jordan, head of insurance at the Bexhill-based Subsidence Claims Advisory Bureau. "A lot of insurance companies run a mile when asked to cover a property that has suffered from subsidence." The Bureau has set up its own insurance scheme, Previously Underpinned Properties, or PUP, for such blighted houses.
Subsidence damage can also be a problem when you want to sell. Mortgage lenders may be reluctant to lend on affected buildings. The Halifax, for example, will consider properties with a history of subsidence if there has been no recent movement, or if a structural engineer certifies that underpinning has done its job. Where the valuer suspects subsidence is ongoing, they will consider lending only if the house is insurable.
All of which may sound depressing, but Mike Crilly remains upbeat. "Climactic conditions are ideal for causing greater amounts of subsidence damage in the long term ... and you can't underpin the entire country. Learn to live with your cracks, as people in the period pre-insurance did. It's probably not as bad as you think."