What a rear, side-return or double-storey extension really costs in South London — honest price ranges and the factors that move them, from a builder who quotes these every week.
| Type | Typical all-in cost |
|---|---|
| Single-storey rear (build cost) | £2,200 – £3,200 / m² |
| Single-storey rear (~20 m²) | £45k – £70k |
| Side-return (~12–15 m²) | £35k – £55k |
| Double-storey rear | from £75k |
| Add bifolds / roof lantern | + £4k – £12k |
Indicative all-in figures (labour + materials) for South London in 2026, before kitchen or bathroom fit-out. Groundworks, glazing and finish level all move the number — a site visit gives you the real one.
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Tell me the rough size and what you want from the space — I’ll give you a realistic figure and timeline.
