How Much Does a Side Return Extension Cost in the UK?
A side return extension fills in the narrow alleyway that runs alongside a terraced or semi-detached Victorian or Edwardian house. Although the footprint gained is modest — often just 2–3m wide — it can transform a dark, cramped kitchen into a large, light-filled kitchen-diner. The key feature is usually a glazed roof (roof lantern, skylight, or full glass roof) that floods the extended space with natural light.
Typical Cost (UK, 2026)
Approximately £1,600–£2,400 per m² including labour and materials
All costs are indicative estimates. Obtain at least 3 quotes from qualified builders. Prices exclude VAT (20%).
Factors Affecting the Cost
Length of return
Most side returns run 4–8m. A longer return means more floor area and proportionally lower cost per square metre.
Roof design
A roof lantern or fully glazed roof is the signature feature of a side return extension but adds £3,000–£8,000. A simple pitched or lean-to roof is cheaper.
Kitchen integration
Most side return extensions are used to extend a kitchen. A new fitted kitchen adds £5,000–£25,000 on top of the build cost.
Party wall
Side return extensions almost always share a wall with a neighbour, triggering the Party Wall Act. Surveyor fees add £1,000–£3,000.
Structural complexity
Removing the original rear corner wall to open up the space requires a steel beam — typically £1,500–£3,500 including installation.
Cost Breakdown
| Item | Basic | Mid-range | High-end |
|---|---|---|---|
| Foundation and groundworks | £3,500 | £6,000 | £10,000 |
| Steel and structural work | £1,500 | £3,000 | £6,000 |
| Roof lantern or skylight | £2,000 | £4,000 | £8,000 |
| Glazing (bifold or sliding doors) | £3,000 | £6,000 | £12,000 |
| External cladding or brickwork | £2,000 | £4,000 | £7,000 |
| Internal finish and electrics/plumbing | £5,000 | £9,000 | £15,000 |
Costs are indicative and exclude VAT. Regional variations apply.
Typical Timeline
10–16 weeks on site
Frequently Asked Questions
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