Side Extension Cost UK 2025: Full Price Guide for Single Storey Side Returns
A side extension — also known as a side return extension — can transform a narrow kitchen into a wide open-plan living space. In 2025, the cost of a side extension in the UK ranges from £25,000 to £70,000 for a typical single-storey project, depending on size, specification, and location. For London projects, add 20–30% to these figures.
This guide breaks down exactly what drives side extension costs, what you should budget, and what’s included at each specification level.
Side Extension Cost by Size (2025)
| Extension Size | Budget Finish | Standard Finish | Premium Finish |
|---|---|---|---|
| 2m x 4m (8 sqm) | £18,400 | £26,400 | £36,000 |
| 2m x 6m (12 sqm) | £27,600 | £39,600 | £54,000 |
| 3m x 5m (15 sqm) | £34,500 | £49,500 | £67,500 |
| 3m x 6m (18 sqm) | £41,400 | £59,400 | £81,000 |
Prices are project totals including structural work, build, and finishes. VAT (20%) additional. London adds 20–30%. Kitchen and appliances excluded unless stated.
What Is a Side Return Extension?
A side return extension fills in the narrow alleyway that runs alongside many Victorian and Edwardian terraced and semi-detached houses. These side returns are typically 1.5–3m wide and extend the full length of the kitchen, creating significant floor space when infilled.
The result is usually a wide, light-filled open-plan kitchen-diner — one of the most popular and value-adding home improvements in the UK.
Not all side extensions are side returns — a side extension can also be built on the flank of a detached or semi-detached house where there’s no existing side return. The cost principles are the same, though flank extensions are often larger.
What Affects the Cost of a Side Extension?
1. Size and Depth
The most significant cost driver. A small 2m-wide side return extension of 8 sqm is a fundamentally different project to a 3m x 6m side extension of 18 sqm — not just in proportion but in structural complexity. Larger extensions often need a steel frame to support longer spanning roofs.
2. Roof Type and Glazing
The roof of a side return extension is where costs vary most dramatically:
- Flat rubber or GRP roof: £3,000–£6,000 — lowest cost, reliable if well-installed
- Pitched roof to match house: £5,000–£9,000 — often required by planners
- Glazed roof (polycarbonate panels): £4,000–£7,000 — affordable but can overheat in summer
- Structural glass roof with lantern: £8,000–£18,000 — maximum light, premium cost
- Full-width glass roof extension: £15,000–£30,000+ — Crittall or aluminium frame, architectural quality
Roof glazing is where side return extensions most commonly exceed budget. If you want a light-flooded kitchen, specify the glazing area early and get firm quotes.
3. Structural Steel
Most side extensions require steel beams to:
- Open up the rear wall of the original kitchen to connect to the new extension
- Span the full width of the extension at roof level
- Carry loads where load-bearing walls are removed
Steel typically adds £4,000–£10,000 to the project depending on the number of beams and spans involved.
4. Kitchen Specification
Most side return extensions are built to create a new kitchen-diner. The kitchen itself is not included in basic extension costs — budget separately:
- Budget kitchen: £5,000–£12,000 (IKEA, Howdens, etc.)
- Standard kitchen: £12,000–£25,000 (trade kitchen with quality appliances)
- Premium kitchen: £25,000–£60,000+ (bespoke or high-end brands)
5. Underfloor Heating
Electric underfloor heating adds £1,500–£3,000 for a typical extension. Wet underfloor heating connected to the boiler adds £3,000–£6,000 but is cheaper to run long-term. Many homeowners prioritise underfloor heating in extensions as it’s much harder to retrofit later.