"As a first step, do the sums," advises Stephen Minchin, MD of the northern estate agent group, Reeds Raynes. "Ask your estate agent to value your property in its current state, and what it would be worth with an extra bedroom, bathroom or garage. Your decision will take into account more than just the financial side, but at least you will have a sound basis of facts and figures to work from.
"A key factor is the state of the local property market," he adds. "If what you are looking for is easily available in your chosen locale, it probably makes sense to move, and do without all the dust and inconvenience that comes with alterations. However, if you have a rather nice one-off property, you love the area, and the property market is such that finding what you want is going to be a painful experience, then staying put and adding to the property may well be the best option."
It is rare for properties to have no leeway for expansion, and if the decision is to stay, a strategy of rectifying the property's "weaknesses" - and ensuring that any work carried out is of good quality - will enhance saleability should you later wish to sell. "If you have three bedrooms, put in an extra one," advises Nick Gable of estate agents Winkworth's in Battersea. "Converting the loftspace into, say, a master bedroom plus bathroom, can be an excellent move." It won't come cheap - perhaps around pounds 20,000: "But in this area and in the current property market, you would likely get your pounds 20,000 back, plus say another pounds 10,000."
The cost of a loft conversion, and the returns, will differ regionally, but there is one crucial factor that applies wherever you live. "The quality of loft conversions is variable," warns Paula Henderson of Velux Windows. "It is vital to make sure it is carried out professionally. When we send out our loft-conversion information pack we also send a list of companies in your area whom we've checked out."
One way of expanding properties is to build a conservatory: "A conservatory is a boon to a property, especially where there is only one reception room, or where there is no view to the garden," says Richard Craig, MD of the independent National Conservatory Advisory Service. Good construction is crucial here, too: "We see cases of conservatories that have come away from the wall of the house. Some builders use just a sealant, when the brickwork should have been jointed in or attached by a fixing plate.
"Incorporate heating so that it's an all-year room, and, as with any extension, the conservatory ought to blend in with the existing property, so that it becomes an integral part of it." When Irwin and Annette Stephens built a conservatory on to their 60-year-old property they wanted to make it look and feel seamlessly part of the house. "We went for wood which we painted a `heritage' shade of green. And we visited architectural salvage yards to find reclaimed floorboards for the conservatory." It paid off. When they put the house up for sale, the estate agent selected shots of the conservatory to tempt buyers.
From Battersea, Gable agrees: "It's a winning option. In this area and in a good market, your conservatory which cost around pounds 15,000 would increase the value by around pounds 20-25,000. In a less bullish one you would still get your money back, and you would have had the use of the conservatory for a few years."
If these grand extension schemes are beyond your ambitions or pocket, there is a modest addition that can be valuable in its own way - a downstairs lavatory. Far better to think small than to become a victim of what most estate agents have witnessed - owners who have had a bad case of the extending bug. After all, you have to keep a sense of perspective. If you buy a small property and spend a small fortune doubling its size, it is hard to argue that a move would not have been the better option.
Mann and Company, Ealing: O181 579 6144. National Conservatory Advisory Service: 0500 522525. Reeds Raynes: 01565 634126. Velux: 0800 003555. Winkworth's: 0171 228 9265.