The problems are so acute that insurance claims for damaged houses have more than tripled this year. By the end of the year insurers expect them to total more than pounds 400m, compared with pounds 125m in 1994.
Worst hit are areas of the south, from Bedfordshire to Southampton, where houses are built on clay. And the trees that give suburbia's Acacia Avenues their character have been their undoing. As the heatwave continued, and no rain fell, trees drew water from the ground, causing the normally moist clay to dry and shrink, and the houses to begin to subside. For a three-bedroom semi, underpinning would cost at least pounds 20,000.
Homes with shallow foundations built in the Twenties, Thirties and Fifties are the worst affected, although Victorian and Edwardian villas have also suffered. In some neighbourhoods, whole streets of houses have been hit, while blocks of flats have also shown signs of subsidence: cracks, shifting window frames, and gaps between roof and brickwork.
In the late Seventies and early Eighties, about 150,000 houses had to be underpinned, costing insurers pounds 2.5bn. Since 1986, tougher building regulations, which demand deeper foundations, have enabled newer homes to survive the record-breaking heatwave uncracked.
In London, the jerry-built houses of 19th-century clerks and artisans which have become the pride and joy of the middle class have been particularly hit. In Kentish Town, north London, for example, Alan Warrington and his family face the prospect of having their home underpinned for a second time.
"Last time it happened we'd wondered if we should get both front and back underpinned, but the insurers quibbled," said Mr Warrington, a publishing executive. "So we did the front and now the back's got problems.
"The cracks started appearing after the summer. The usual explanation is that trees are the problem but ours have all gone. So now we have to face the army of surveyors with their different arguments about what to do next. What happens to what, for most people, is their greatest asset is in their hands."
In nearby Islington, secretary Helen Holden and her flatmates have watched the cracks in their Victorian house get bigger and bigger. Now they can see the outside world through the wall, and even into next door's bedroom. "We'd realised the house was moving," she said, "but since the summer it's got much worse. Some of the doors don't fit properly, and part of the ceiling fell in.''
While homeowners can claim on their insurance to pay for repairs, she and her friends have to rely on spraying Polyfilla into the cracks. They are tenants of a housing co-op which has leased the property from Islington council. The house already needed repairing and the council lacks the necessary funds.
Insurers are worried that matters could be even worse next year. "A long dry period followed by a mild winter increases the risk of subsidence," said a spokesman for the Association of British Insurers. "One half of that equation is in place."
Experts believe that treatment of trees is the answer to the problem of sinking, cracking buildings. A recent survey by the government-backed Building Research Establishment found that 79 per cent of all subsidence claims on shrinkable clay soils were due to the extraction of water by trees or shrubs close to the property. A mature deciduous tree draws up to 50,000 litres a year from the soil - the equivalent of what a family of four would use in three months.
Willows are the worst offenders. Rob Hooker of the Subsidence Advisory Bureau says the best place for them is at the bottom of the garden away from the house. Other large water users should also be kept away (see accompanying chart). He advocated tree surgery, such as pollarding and pruning every five years. "We're not philistines; we don't want to go round pulling trees down. If the tree is there before the building, the tree stays."
But for householders who have chopped down their trees, made their insurance claim, and had their home underpinned, their difficulties may not yet be over. Professor Malcolm Hollis, a building surveying expert at Reading University, says underpinning may cause further problems. "If a building might drop in a hole, the last thing you should do is add load to it," he says. "What is now preferred is a 'stiffening beam' put around the building. It's less disruptive, the weight sits uniformly, and it's a third of the cost.
"Put in a lump of concrete when you're underpinning and the tree roots will work round it."Reuse content