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How much does a house extension cost in London in 2026?
Indicative cost ranges for side-return, wrap-around, rear and double-storey extensions across London — based on real Prime Build projects in Ealing, Richmond, Chiswick and Croydon.
Prime Build Design · 23 May 2026
What drives the cost
The biggest single driver in a London extension is the finish level. A standard finish runs £2,800–£3,800/m², premium £3,800–£5,200/m² and luxury extensions in conservation areas often exceed £6,000/m².
Cost by extension type
- Side-return (single storey): £80,000 – £140,000
- Wrap-around (single storey): £120,000 – £220,000
- Rear extension (single storey): £75,000 – £165,000
- Double-storey extension: £180,000 – £380,000
Hidden costs to budget for
Many extension quotes exclude planning fees, party-wall surveyor instructions, building-control inspection fees and structural-engineer designs. Allow 8–12% on top of the headline build cost for these.
How to keep costs sensible
- Lock the scope before tendering — late changes carry premium pricing
- Choose a builder with in-house joinery and MEP — sub-contractor chains drift on cost
- Ask for a fixed-price contract with staged payments tied to milestones
- Get a free consultation before you commit
This guide is indicative. For an instant ballpark, try our AI Cost Estimator — or book a free site visit for a tailored quote.