What is the difference between single-storey and double-storey extension cost?
A double-storey extension costs less per square metre than a single-storey one — typically £2,800 to £4,000 against £3,000 to £4,500 — because the foundations, roof and groundwork are shared across two floors. The total is higher, but the cost per square metre is lower.
The expensive elements of an extension are at the bottom and the top. Foundations, drainage and groundwork are a large fixed cost, and so are the roof structure and covering. Adding a second storey reuses both, so the additional floor costs mainly its walls, floor structure, windows and internal finishes — which is why the rate per square metre falls.
In practice a single-storey rear extension of 20m² at £3,500/m² is around £70,000. Doubling it to two storeys and 40m² does not double the cost; it typically lands around £120,000 to £130,000 rather than £140,000. If you need the space and the planning position allows it, going up is the better value decision.
The catch is planning. Permitted development for two-storey rear extensions is much tighter — a maximum of 3m depth, no closer than 7m to the rear boundary, and not permitted at all on a terrace in many circumstances. A two-storey extension in Fulham usually needs full planning permission, and the impact on neighbours' light and outlook becomes a material consideration in a way a single storey rarely does.
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