The job: to convert a roughly 300 sq ft attic space in a 75-year-old house. Attic conversions are all the rage at the moment, as a way of extending living space without the expense of moving home. And everybody in the house will stake their claim to the new space the minute you mention you're considering this move.

You may want it as a study, office, or an extra bedroom. (You will not, however, be able to describe it as a bedroom if you ever come to sell the house because of building regulations governing floor-to-ceiling heights of habitable areas, and possible fire regulations.) Teenagers will want it as a cosy lair; younger children will see it as a playroom.

But where to start? There is a forest of companies advertising attic conversions in the Golden Pages, the first stop in my hunt - and they mostly begin with A: AB Builders, A & B Carpentry & Roofing, ABC Attic Ladders, The Attic Shoppe. It seems to go on for pages without creeping beyond the letter c. Attic Conversion Services in Harold's Cross has a solid looking display ad, and more importantly, actually answers the phone - calls to half a dozen other numbers yield answering machines or simply ring out. And the woman in Attic Conversion is full of solid, helpful information.

"We only do full conversions," she says, explaining "the roof is restructured with steel beams and the floor is hung from them, not put resting on the existing floor boards. That's the safest way to do it and the only job that will be certified by an architect."

Not that anyone will necessarily check. "No, you don't need planning permission unless you put on a dormer or gable window or roof windows to the front; no planning permission is needed to put windows in at the rear." However, if your house is listed as a protected structure, check with the planning authorities.

How many windows? This has a more mathematical answer than you might think. "The light should be more than 10 per cent of the room. If the floor area is 120 sq ft, the windows would have to come to 15 sq ft, but we'd have more than that."

But down to brass tacks: how much will this all cost?

"For a standard three-bed semi with a gable end, we'd start at around £12,000; we try to do proper stairs, as a continuation of the existing stairs. An en suite would add around £3,000."

As with all companies doing attic conversions, she can't give more specific information without seeing the house, but says reassuringly that it might not take as long as I think to go from ground zero to finished job. It would take about 12 to 18 days to convert the attic of a standard three-bedroom semi, and more time if installing an en suite. But you shouldn't have to leave home while it's being done. "People generally don't. The first few days, when you're breaking through, are the dirtiest."

This conversation has thrown up a few of the basic relevant questions concerning attic conversions which I can't answer - the most basic being "What kind of roof do you have?" Is it a hip roof? Pitch roof? An architect friend tries to explain some of the basics to me. The cost of an attic conversion will depend not just on the kind of job you want done, but on the age and state of your roof. And the older your house, the easier it might be to convert: up to around 25 years ago, roofs were built with cut timber; since then, they have generally been constructed with prefabbed timber trusses, which can break more easily if you try to cut and change them.

He also warns that whatever job you do, you need to be careful about having easy escape from the attic area in case of fire. And although it's true that you probably won't need planning permission for most attic jobs, you should comply with building regulations - this will be important if/when you come to selling your house.

The other business is the matter of floor area - seemingly this will be determined by the height of your roof, bearing in mind that the walls at the side can't be lower than 3 ft 10 in.

Martin Lynch, a builder who has started to specialise in converting attics, highlights some of the choices you need to make in light of the use you're going to make of the area. There's obviously a world of difference between putting down a simple floor in an attic to use it for storage and going the whole hog, putting in en suites and proper stairs and so on.

"The stairs alone can be pretty expensive, especially if you want them to match the existing stairs - they could be £4,000. But £12,000 is around the right mark; it depends of course on the condition of the existing timber, and whether you need supports, and what it will be used for - if it's going to be a kids' playroom and you might have 10 kids jumping on the floor you'd have to provide for the maximum load."

An en suite could add another £3,000, he confirms. So might an attic conversion add up to as much as £20,000? "That would be a pretty good one," he says reassuringly, implying that the final cost should come in at less." Martin Lynch, like Attic Conversions, says that doing the job isn't as disruptive as it might appear. The whole job could take about a month, water and electricity supply should only be interrupted briefly while tanks are moved, and plumbing installed for an en suite.

But like most other people in the building industry, he's pretty busy. The earliest he would be available to do a work is October.

Tony, at the Attic Man, will also send brochures giving details and pictures of jobs already done, plus phone numbers of people who've agreed to give him references.

His first question has been the one that I'm still not sure if I'm answering properly, but he concludes that we're talking about a hip roof. He talks me through everything from a budget conversion, which simply involves reinforcing the existing ceiling through to a conversion involving an en suite. Basing his estimates on a floor area of 200 sq ft, he says the budget job would cost around £7,000. A proper floor - he also uses the example of a troop of children leaping up and down, causing vibrations in the wall - would have to be suspended from steel beams, and would cost, he estimates, just under £9,000. He is including the cost of basic "proper" stairs (ie., not a ladder), as well as sockets, lights, moving the tank into the eaves, fitting smoke alarms, and two Velux windows in this price.

An en suite - consisting of a lavatory, wash-hand basin, shower or bath, pump units and another Velux - would add around £4,000 to the cost, he says, bringing it to about £14,000.

The verdict: for less than £20,000, you should be able to get a good attic conversion that could give you valuable extra space - and it will pretty certainly work out cheaper than moving house.