Basement Conversion

Converting or creating a basement level beneath an existing property. Involves extensive structural work including underpinning the existing foundations, waterproofing (tanking or cavity drain membrane), and new drainage connections. Planning permission is typically required for a basement extension because it involves significant work outside the existing footprint. Basement conversions are expensive - typically £3,000–£4,000 per m² - but add significant floor area in densely built areas.