Judicious renovations were once the best way to add value to your home - and in a rising market often came just before an owner sold up for a fat profit. But as house prices deflate, the expenditure of vast sums on professional “improvements” is, to many homeowners, more like a gamble than a money-spinner.

With the 20 per cent rise in costs over the past two years it is even harder to make the sums add up. The latest Property Makeover Price Guide, produced by the Royal Institution of Chartered Surveyors (RICS ), puts most blame on the rising cost of transport and building materials.

The price of a loft conversion is up 21 per cent: you should now expect a new 6m x 5m attic room, with two Velux windows, to cost £17,000. But the price increases are even more acute for basement conversions, up 25 per cent to about £10,000.

Extensions are 22 per cent more expensive: budget £22,500 to £28,000 for a 4m x 4m single-storey addition to your home, or £18,500 to £23,000 (up 22 per cent) if you are willing to make do with a conservatory.

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Even the price of small, less complex jobs is piling up, adding to the squeeze on householders' cash. RICS reports that the cost of sorting out roof tiles is up 17 per cent. Resecuring six tiles will now set you back £310. Replacing a 2m stretch of cracked brickwork is up 16 per cent and will now cost £240 to £380.

The price of repairing damp-affected walls is also up 16 per cent, to £83-£135 for an area up to 5 sq m. And new double-glazing on a window will cost £1,040 to £1,930.

RICS also gives a warning that an exodus of skilled tradesmen from the UK - many of them Eastern European nationals returning home - is worsening the price crisis. But the effect is being felt differently throughout the country. Buildstore , the self-build and renovation centre, says that the average day rate for a tradesman is most expensive in London, at £194, and Scotland, at £190, and cheapest in the North West, at £158, and Wales, at £162. The labour costs for a plumber are most expensive, followed by electricians, while decorators and bricklayers remain relatively inexpensive.

Just as headlines about food prices has sent homeowners scurrying to the allotment, this latest flurry of depressing statistics is likely to prompt a nationwide rummage among discarded tools in sheds. And future homeowners will be likely to be reversing the effects for decades to come.

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