You have spent the past six months staring at that crack in the plaster or the damp bit in the corner and it is not getting any better. You have asked a couple of local builders to have a look (and anybody else who is passing), but you cannot get definitive answers as to why it is there, whether it is serious and what to do. So, how much would it cost to fix, and who do you trust? Sound familiar?

Most of us have a few cracks or the odd damp patch; Helen and Tom Kenyon have a whole house of problems. Their two-bedroom, 18th-century thatched cottage in rural Wiltshire, surrounded by a wild garden with ancient apple trees and hens clucking in the long grass, is in dire need of renovation.

They bought it three years ago after the old lady who had lived there all her life died. “Inside, it was ghastly – horrendously damp and freezing,” Helen says. “The window frames were rotting and there were 12 different strips of wallpaper in the sitting room.”

Yet the fireplace, the window seats, the narrow wooden staircase and the bucolic position on a gentle slope with views down the valley were everything they’d dreamt of. “We fell completely in love with it,” Helen says. “We are incredibly happy here.”

The house still needs a lot of work: it has a 1950s kitchen and no central heating. And the couple, who started renovating it themselves, reckon they need 10in of thermal insulation in the loft, but believe the resulting lack of air circulation could rot the thatch. What should they do?

Tom, 40, a management consultant, and Helen, 36, a garden designer, fired off an e-mail to the Historic Building Advisory Service (HBAS; www.historicbuilding.co.uk), a new website offering advice to the owners of period properties. Within days, they received their answer: make sure that insulation is kept away from the eaves to allow a ventilation route on all sides of the roof. The advice explained the size of gap needed and how a proprietary void former could be used – arming the Kenyons with sufficient knowledge for an informed conversation with the builder. (Admittedly, the job has not yet been done, so the ultimate test is still to come.)

Nearly one in four people in England live in homes that were built before 1919, according to the English House Condition Survey of 2003, and 8.5m homes in the UK were built before 1945. We love their original tiled floors, exposed beams, stained-glass panes, even the bowed ceilings – but, once we have fallen for the sash windows and the inglenook, and parted with a lot more cash than we would have done for the equivalent space in a newer home, do we know how to look after our most valuable asset when things go wrong?

“The main mistake people make with old houses is expecting them to behave like new ones,” says Robert Hill, a chartered surveyor by training and the man behind HBAS. “They’ve seen the pictures in Homes & Gardens, with everything looking perfect for the photographs. But you have to work with an old house all the time: it doesn’t remain static, so you can’t ignore it. People end up devaluing an old home by allowing inappropriate materials to be used. And owners do get ripped off.”

With the backing of the Royal Institution of Chartered Surveyors, which is regulating the site, Hill, 56, has amassed a team of experts to answer questions from owners of everything from listed barns to 1930s bungalows.

For an annual subscription of £55, homeowners will receive answers to two questions (however much research is involved), a growing list of frequently asked questions and case histories, e-newsletters and, should you need to ask more than two questions, e-mail access to specialists for a fee (from £37 upwards, depending on the question). Site visits can be arranged at £75 an hour.

With the plethora of consumer sites now available online, couldn’t you get most of your problems solved free simply by surfing the net? Up to a point, Hill concedes. “The difference is, we can give a tailored answer to a specific question relating to a specific building, rather than more general advice,” he says. “And you can send us pictures of your problem via a portal on the site.”

Not everyone is convinced by the push-button solution. “I don’t think there’s any substitute to going out to look at a problem,” says David Mills, of the Bath-based independent building surveyors DR Mills and Associates. “If you were ill, you wouldn’t want to be diagnosed via a website.”

He warns that you could end up wasting money on repairs if you have not fully understood the causes, citing the case of a mews cottage he looked at recently. “There were no readings on my damp meter except in one position, when it shot up to the maximum. The other side of the wall was a garage. It was a mystery until we found a hairline crack on the side of a main that was hidden from view, which had been gradually leaking.”

I thought I would test the site myself with a nasty problem that I have not solved in the 10 years I have owned my 200-year-old house in Hertfordshire, despite asking two local builders and a professional painter and decorator. Why, I asked, do I keep getting a discoloured bubble effect on one exterior white-painted brick wall, and what should I do about it?

The answer pinged back. The solid brick walls of my house have probably been painted with a modern plastic or polymer-based paint, suitable for new homes, but not 200-year-old bricks, which must be allowed to breathe. This north-facing wall has absorbed moisture from the ground (despite the damp course) and the bubbles and discoloration are salts building up behind the paint. The walls should have been painted with white limewash, which would let the bricks breathe.

Ah. It appears I am one of those ignorant owners who allow unsuitable materials to be used and end up devaluing their home.

A virtual way to decorate, too...

Damp patches and cracks fixed, but stuck on how to do the place up? Keep it simple and white, or splash out on this season’s floral-print wallpaper? Go to mydeco.com, where you can upload a photograph of the room you’re working on, drop in your new ideas over the current walls and see how it will look. The website, launched by the founders of lastminute.com, also provides a “try before you buy” service, offering tips from designers that cover most budgets and styles, and 3-D tools for creating a digital plan of a room. By selecting an interior or exterior job from a dropdown menu, for example, you can move a door, replace a staircase or change the kitchen worktop. The site will give you a full breakdown of the likely cost.

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