How can that nice bloke who has spent the last few months transforming your home suddenly become so difficult? Penny Jackson highlights the problems many home owners face as a building job ends, and offers some tips.

Never mind all those cups of tea and friendly chats; now he has the money, he can't get out of the house fast enough. What happened to his concern about the wonky tiling, untidy wiring and possible leak in the new conservatory roof? They disappeared down the garden path as rapidly as the cheque.

The story is familiar. Not the one about the builders from hell, but the ostensibly reliable company that may even have come with a personal recommendation.

Everything goes swimmingly until, shortly before the work is finished, payment is requested. "We've only got a couple of days' more work, the lads need paying ..." So you settle up, reminding yourself that he isn't a cowboy builder, and anyway he's been really good about feeding the cat and talking football to the kids. Up to that point the work has been satisfactory; you trust him to finish the job to the same standard.

But anyone embarking on home improvements for the first time finds out that the waters can be treacherous. Deborah Brunero can still get upset when she recalls how miserably her loft conversion ended.

To make matters worse, Deborah was pregnant and finding everything a struggle. She says: "We had been scrupulous in paying the agreed instalments, and were very happy with the work. But it all went wrong at the end. The main guy disappeared and we had a mish-mash of people."

"In the last week he came to ask for the final payment of pounds 1,500, saying they would be finished in a few days. I couldn't think of a good reason why not, so I gave it to him. From that moment he lost interest. Even though, among other things, we had serious problems with the plumbing, he wouldn't come back.

"I found it extremely upsetting, and still do. I trusted him, and treated him as I would expect to be treated myself. It got very unpleasant. I would have recommended him to anyone before that. I wish I'd not paid him until we were completely happy."

It was a hard lesson for the Bruneros. As is common for a large job, they had agreed to staged payments for the job, paying an initial deposit of 10 per cent, 30 per cent when work started, another 30 per cent on day eight, a pre-plastering payment of 25 per cent, and the final 5 per cent on completion.

The advice from the Office of Fair Trading is that in general a deposit should not be paid to a contractor. Even if it seems fair to pay in advance for specialist materials or fittings, it is better to order the goods yourself and have them delivered direct to you.

The National Consumer Council makes the point that any reputable company should have lines of credit from suppliers, and not ask for money in advance from customers. Since guarantees can be worthless - Deborah Brunero's was for only five years - there is a good case for keeping back an agreed amount for a reasonable period, which could range from two weeks for decorating work to 12 months for a central heating system.

Another issue that can cause problems when building work has been finalised, is the list of extras. Agree the costs as they arise, advises the OFT. One elderly couple were not so endeared to their builder after he billed them for what they thought had been his kindness in tending to an emergency while working in the house.

Certainly, all consumer groups recommend that a credit card is used where possible, because of the extra protection it offers. This can also apply to any loans linked to the work.

Of course, all protection goes out of the window if transactions are made in cash for a cheaper job. With "nothing in writing" as its guiding principle, this provides a field day for the unscrupulous. Anyone who does pursue a claim could find it embarrassing to explain in court why there is no documentation, says Gordon Powell, from Westminster Council's trading standards service.

A schedule of work and costings is to everyone's advantage; after all, there are plenty of customers who will find any excuse not to pay what they owe. Emma Perring, who employs builders and decorators regularly through her company Perring Designs, applied the same approach to work on her family home, a barn conversion.

"We drew up a detailed work plan which our builder stuck to well. It's a good way of keeping pace with the work, especially if you are not there to oversee it. You have to take the bull by the horns, and insist on employing someone only on that basis."

In her case, the local reputation of their builder meant that his guarantee was worth the paper it was written on. Everyone has their own way of reducing the chances of a final showdown. One London couple frequently use their second home as a carrot. "A week's holiday in the Lake District if the job is done entirely to our satisfaction. Two if you do some repair work while you are there." It works wonders.