Side return & side extension cost UK 2026
"Side return" and "side extension" get used interchangeably, but they cost differently because they're different jobs. A side return fills the narrow outdoor alley beside the back of a Victorian or Edwardian terrace — a small area, big impact. A side extension uses the full gap at the side of a semi or detached house and behaves like a normal single-storey extension. Here's what each costs in 2026.
Side return infill
Typically 4–7 m² of new floor area, added to the existing kitchen. Because the fixed costs — a new side wall, a valley or mono-pitch roof tying into the existing one, structural steel to remove the old external wall — are spread over a small area, the effective cost per m² is high.
| Element | 2026 cost |
|---|---|
| Side return shell (4–7 m²) | £15,000 – £30,000 |
| Effective cost per m² | £1,900 – £2,200 (higher on very narrow infills) |
| Roof lantern or glazed roof | £2,500 – £6,000 |
| Structural steel (removing the old wall) | £1,500 – £4,000 |
| Typical all-in (with glazing, fees, VAT) | £25,000 – £45,000 |
Full side extension
On a semi or detached house, a side extension uses the gap between the house and the boundary. It's priced like a single-storey extension at £1,900–£2,600 per m² standard spec.
| Size | Build (ex-VAT) | All-in guide |
|---|---|---|
| 8 m² (small side extension) | £15,000 – £22,000 | £22,000 – £32,000 |
| 12 m² | £23,000 – £32,000 | £33,000 – £46,000 |
| 18 m² | £34,000 – £47,000 | £48,000 – £67,000 |
| 25 m² (side + rear wrap) | £50,000 – £70,000 | £70,000 – £98,000 |
Regional adjustment
London +25–40%, South East +10–25%, South West +5–15%, Midlands around average, North England −5–10%, Wales −10–15%. Side returns are most common in London and the South East, where the regional premium applies.
Planning permission
Single-storey side extensions can be Permitted Development if they're no wider than half the original house, single storey, under 4 m high, and finished in similar materials. Side returns usually meet these limits; wider side extensions, two-storey side extensions and anything on designated land need a full planning application. Building Regulations approval is always required, and a party wall agreement is common because the work sits on or near the boundary.
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Frequently asked questions
How much does a side return extension cost in the UK in 2026?
A 4–7 m² side return infill is around £15,000–£30,000 for the shell, or £1,900–£2,200 per m². All-in with glazing, fees and VAT, £25,000–£45,000.
What is the difference between a side return and a side extension?
A side return fills the narrow alley beside a terrace. A side extension uses the full side gap of a semi or detached house and costs £1,900–£2,600 per m² like a normal single-storey extension.
How much is a small side extension?
A 6–10 m² side extension is typically £15,000–£28,000 for the shell. Narrower additions cost more per m² because fixed costs spread over less area.
Do I need planning permission for a side extension?
Single-storey side extensions can be Permitted Development within width, height and materials limits. Wider or two-storey side extensions need a full application. Building Regulations approval is always required.
Is a side return extension worth it?
On a terrace it's one of the highest-impact changes for the money and usually adds more value than it costs — provided the house isn't already at the ceiling price for the street.
A quick note
Indicative 2026 UK figures from published cost guides. Narrow infills vary a lot with roof and steel design — get three like-for-like quotes. See our full disclaimer.