The honest answer: it depends — but here are real numbers
A 4m x 4m single-storey rear extension in London will typically cost between £60,000 and £95,000 in 2025, including VAT. The range is wide because extensions are bespoke construction projects.
The main cost drivers
1. Size and footprint
Rule of thumb for London in 2025: £2,500–£3,500/m² for a standard single-storey extension; £3,000–£4,000/m² for a double-storey; £4,000–£6,000/m²+ for high-specification finishes and bespoke glazing.
2. Specification and finishes
Nothing moves the price like specification: bi-fold or sliding doors (£5,000–£15,000+), underfloor heating (add £80–£150/m²), structural glazing or roof lanterns (£3,000–£10,000+ per unit).
3. Structural complexity
If you have clay soils (common in London), deep strip or mini-pile foundations can add £8,000–£20,000 to groundworks costs.
4. Party wall matters
If your extension is within 3m of a neighbour's foundation, the Party Wall Act applies. Budget £1,000–£3,000 per adjoining neighbour for Party Wall Awards.
London vs rest of UK
London and the South East command a price premium of 20–40% over equivalent work in the Midlands or North.