Most basement flats are in converted houses, and you often get more space for less money than you'd pay for a flat on an upper floor. This is a real advantage in central London: "First-floor flats tend to be the most expensive," says Simon Edwards of Winkworth's estate agents in London, SW1. "That's followed jointly by the second and ground floors. A basement will typically offer more space - they don't have communal parts for a start - and while an 800-foot square non-basement flat costs around pounds 240,000, a basement of similar size would be pounds 170,000-pounds 200,000."
Many basement flats have their own entrance. Older properties often have a useful storage area under the front steps of the house, which generally belongs to the basement. And there is often a patio or garden at the back.
Gilly Wall lives in a basement with her partner Greg and their son George. She says: "It was definitely hard-going pulling a buggy up the basement steps every day, but once we got over that phase the garden has played a big part in our staying. We are lucky that it is a decent size, and very mature. With a lively five-year-old, it is an absolute boon."
So don't rule out basements when you are house-hunting, but be aware of their peculiar problems. Peter Trotman of the Building Research Establishment, an independent body, says there are two crucial areas for consideration - damp and ventilation. Trotman says the only way to tell whether an individual flat is going to hold out against damp in the long term is to look at the work that has already been carried out. That means you'll need to spend money on a good survey.
Lenders often like to see timber and damp reports on basements, but be aware that there is a lot of debate in the building world about the usefulness of electrical moisture meters. The readings from these can be unreliable, says Trotman. "They should be used as a preliminary tool, they will not tell you whether the building is damp or not. You need to sample the material." He also recommends finding out what soil the property is built on. A lot of London homes are built on clay, which is notorious as a cause of subsidence.
Adequate ventilation can also be a tricky issue because extra glass at ground level causes security problems. Roy Emslie, crime prevention officer with the Metropolitan Police, says: "Because they can be darker than other flats, residents unsurprisingly try to make the most of the incoming light. Often a door is half or three-quarters glazed to let in more light, and French doors are popular. Make sure you have locks which meet recognised standards." An alarm is also a good visible deterrent, and Emslie suggests planting the thorniest, prickliest plants you can find around your perimeter fences.
Home insurers don't automatically charge you more if you live in a basement. "The crucial deciding factor is location," says Malcolm Tarling at the Association of British Insurers (ABI). And if insurers do hike their prices for basement-dwellers, it may not be because of the burglary risk. "There have been cases where basements have been flooded when there has been a water mains burst," says Tarling.
The best bet, if you can afford it, is to buy a flat on the two lower floors of a house. "You can be sure of getting lighter, brighter rooms upstairs, where you can put the sitting room, with the bedrooms below, while still having the outside access that goes with the property," says Daniel Todary, manager at Austin Daniels estate agents in Islington. But the price is prohibitive: Austin Daniels recently had a garden maisonette for pounds 325,000, whereas a basement in the same street cost pounds 150,000.
If you do move into a basement, prepare to be much closer to wildlife than you would be in an upstairs flat. It comes as no shock to basement- owners to find spiders, frogs or even rats in their home, as well as next- door's cat.