UK extension costs · 2026
Side return extension cost in the UK (2026 prices)
A side return extension fills in the narrow alley beside the back addition of a Victorian or Edwardian terrace, usually to widen a cramped kitchen. Areas are smaller than a rear extension, but the cost per square metre is higher because access is tight and structural work is more involved.
Typical side return extension cost by size
| Size | Indicative cost |
|---|---|
| Narrow infill (about 8 m²) | £13,000 – £16,500 |
| Typical side return (about 12 m²) | £19,500 – £25,000 |
| Long side return (about 18 m²) | £29,500 – £37,500 |
| Wrap-around side return and rear (about 28 m²) | £45,500 – £58,000 |
Indicative only, based on typical UK build rates for a standard finish. Your own quote depends on your property, access and specification.
Why side returns cost more per m²
Materials and waste often have to be carried through the house, the working space is narrow, and removing the old side wall usually needs steel beams and temporary propping.
Fixed costs such as scaffolding, welfare, skips and professional fees are spread over a much smaller floor area, which pushes up the rate per square metre.
Wrap-around extensions
Most homeowners combine the side return with a modest rear extension to create a wrap-around. The extra rear depth is comparatively cheap once the site is already set up, and it transforms the layout.
If a wrap-around is a possibility, design it now even if you build in stages, so the structure and drainage allow for it.
What affects the price most
- Floor area in square metres — the single biggest driver of the total price.
- Access to the rear or side of the property for machinery, deliveries and waste.
- Ground conditions, drainage runs, manholes and any nearby trees affecting foundation depth.
- Structural openings: a wide steel beam into the existing house costs more than a standard doorway.
- Glazing choice — bifold or sliding doors, roof lanterns and large windows add several thousand pounds.
- Internal fit-out: kitchens, bathrooms, underfloor heating, flooring and decoration.
- Professional fees: architect or designer, structural engineer, building control and any planning application.
- Where you are in the UK — London and the South East typically run 10–20% above national averages.
Frequently asked questions
- How much does a side return extension cost?
- A typical 12 m² side return usually costs around £22,000 to £32,000 for a standard finish, with wrap-around schemes running considerably higher.
- Do I need planning permission for a side return?
- Infilling a side return is often permitted development on a house, but conservation areas, flats and listed buildings need consent. Building regulations always apply.
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