requirements, as well as a possible levy on basement construction. So is it all worth it? Property experts are beginning to sound sceptical. "Many people who have done such extensions assume that the value of adding this space is proportionate to the rest of the house," says Oliver Hooper, director at high-end buying advisors Huntly Hooper. "But due to its limited use, the value diminishes the further you go down, while the costs of doing so increase." And the problem remains that spaces below ground are always going to be dingy and confined to particular functions. As Tom Tangley, partner at Knight Frank, says: "If your typical London terraced house is 20ft wide, and you dig out a double basement, all you are ending up with is a deep, narrow corridor. It's very difficult to ventilate and light -- so what's the use?" Still, he does admit a soft spot for a property on their books behind Kensington Square, currently on the market for 15.95m, which features a swimming pool whose floor rises up to become a ballroom. But the tide is beginning to turn, according to Lindsay Cuthill, a director at Savills estate agents: "When this trend began five years ago, there were all sorts of amazing media rooms with surround sound and banked seating, designed for friends to come round," he says. "But people are realising they're not using them like that. Friends aren't coming round. They're probably great if you're 13 years old, but it seems they