In 40 seconds
Groundworks on a typical UK project usually costs roughly £5,000–£50,000+, with the foundation work commonly making up around 10–15% of the total build cost. Strip foundations run roughly £130–£200 per linear metre for standard domestic depths, so a three-bedroom house often lands somewhere around £10,000–£18,000 depending on perimeter and ground. Measured by area, a single-storey extension base of around 20m² typically falls in the £4,680–£9,500 range, often around £6,000–£7,500 outside London and the South East. The honest answer is always a range, because it depends on your ground conditions, the foundation type, access and how much soil has to be excavated and carted away.
Most groundworks guidance is published by firms doing the work, so the numbers tend to be optimistic and the variables glossed over. The pages below give honest cost ranges for foundations, excavation, site clearance and drainage, and explain what actually moves the price — before you take a single quote.