In 40 seconds
Converting an existing cellar into usable living space typically costs around £1,200–£2,250 per square metre, which usually works out at roughly £25,000–£75,000 for an average space depending on size, waterproofing and finish. Lowering the floor to gain head height pushes that to about £2,000–£3,500 per square metre, while digging out a brand-new basement is far more involved at roughly £3,000–£5,000 per square metre, with whole-project costs commonly running from £150,000 and well beyond on larger or London projects. Converting an existing cellar often needs no planning permission, but creating a new basement or adding a lightwell usually does, and every basement conversion needs Building Regulations approval. The honest answer is always a range, because it depends on whether you are converting or excavating, your head height, waterproofing and ground conditions.
Most basement-conversion guidance is published by the firms doing the digging, so the figures tend to be optimistic and the rules glossed over. The pages below give honest cost ranges, explain the cost per square metre, weigh whether a conversion is worth it, compare a basement with an extension, and set out the planning and building-regulations rules — before you take a single quote.