How Much Does a Side Extension Cost in 2026? (UK)
A UK side extension in 2026 typically costs £25,000–£70,000 for a single-storey infill, rising to £55,000–£140,000 for two storeys with quality glazing. Price is driven by width (how much of the original side wall you build into), storey count, roof type and kitchen / bathroom fit-out. A narrow 1.5m single-storey infill runs £25k–£38k; a 3m double-storey side return with a master bedroom above and bifolds below can reach £130k+. This guide shows what your budget actually buys, with worked examples, width-by-width pricing, and the hidden extras London and South-East homeowners get caught by.
Planning the work? Check what it usually costs, or work out your own estimate. The free quote form is at the top of this page.
How much is a side extension in 2026?
Typical 2026 UK side extension costs:
- Narrow single-storey (1.0–1.5m) — £25,000–£38,000
- Standard single-storey (1.5–2.5m) — £35,000–£58,000
- Wide single-storey (2.5–3.5m) — £52,000–£75,000
- Double-storey narrow (1.5–2m) — £55,000–£85,000
- Double-storey wide (2.5–3.5m+) — £85,000–£140,000+
Most UK side extensions are single-storey infills on semi-detached or end-of-terrace homes, used to widen a kitchen or add a utility/snug. Double-storey adds the strongest value uplift but typically needs planning permission and a wider plot.
Side Extension Cost by Width & Storey
Width sets the ceiling on everything — floor area, glazing span, kitchen layout and roof structure. Here's what each width really buys in 2026.
1.0–1.5m — Narrow infill (single storey)
Tight infill next to the kitchen — adds 6–10m². Enough for a utility, downstairs WC, or pantry. Common on Victorian terraces where the original side passage is now redundant.
1.5–2.5m — Standard single-storey side
Most common single-storey side extension — 12–18m² added. Lets you widen the kitchen, add a dining nook, or push the living room sideways. Typically falls under Permitted Development on semi-detached homes.
2.5–3.5m — Wide single-storey side
18–25m² added — enough for a full kitchen-diner extension or a separate snug + utility. PD rules cap side at 50% of the original side width; planning is often needed at this scale.
Double-storey narrow (1.5–2m)
12–18m² downstairs + matching footprint upstairs. Adds a second bathroom or 4th bedroom — one of the strongest value-uplift moves on a 3-bed semi.
Double-storey wide (2.5–3.5m+)
25–35m² over two floors — kitchen-diner downstairs, master suite or two bedrooms above. Effectively converts a 3-bed semi into a 4-bed family home; planning permission essentially always required.
What Actually Drives Your Bill
Two same-width side extensions on the same street can land £20k apart — same footprint, very different price. Here's what explains the spread.
1. Storey count & roof tie-in
Going up a second storey roughly doubles cost per m² of footprint because you double the structure, halve the roof simplicity, and add render, fascia and bedroom fit-out. Tie-in to existing roof (vs flat with parapet) adds £4k–£9k.
2. Planning vs Permitted Development
Single-storey side extensions on semis up to 50% of side width usually fall under PD. Anything wider, two-storey, or in a conservation area needs planning — adds £1,500–£3,500 in fees and 12–20 weeks to the timeline.
3. Side-passage access
No side passage? Materials must be carried through the house and dust-protected — adds £3,000–£8,000 in labour and protection. End-of-terrace and detached are cheapest. Mid-terrace with no side passage is most expensive.
4. Party Wall Act
If your side extension goes up to or near the boundary with a neighbour, you'll trigger the Party Wall Act. Budget £1,500–£4,000 in surveyor fees; allow 2 months for notices and any Award.
5. Glazing spec
Standard windows £1,500–£3,500 added. 3–4m bifolds or sliders £8,000–£15,000. Lantern + bifolds £14,000–£22,000. Half your spec budget can disappear into glazing.
6. Location
London labour runs 30–45% above national average; South East 15–25%. Midlands and North sit at the national average. Scotland adds 5–10% on imported materials.
Side Extension Cost by UK Region
Based on real project data for a standard 2.0m × 5.0m single-storey infill (~10m²) with mid-range spec.
Worked example (illustrative): Single-Storey Side Extension in Bristol
3-bed semi, BS6. 2.2m × 5.5m side infill creating a 12m² utility + open kitchen widening. Completed Feb 2026.
| Architect, structural eng, Building Regs, LDC | £4,200 |
| Party wall surveyor (one neighbour) | £1,900 |
| Foundations & slab | £4,400 |
| Steel frame & structural carpentry | £6,600 |
| Walls, warm flat roof, EPDM, render | £7,800 |
| 2.4m bifold + 2 rooflights | £5,400 |
| M&E first & second fix (UFH, rewire, plumbing) | £4,900 |
| Kitchen extension + utility joinery | £8,600 |
| Floor, finishes & decoration | £3,000 |
| Total (10 weeks on site) | £46,800 |
Valuation uplift: £375,000 pre-works → £435,000 post-works (Savills BS6 comparable evidence, March 2026). Net value-add £13,200 after cost, plus a useful utility / boot-room the family use daily.
Common Questions
Related Guides
More cost, planning and comparison guides to help you plan the right extension.
Side Return Extension Cost 2026
The Victorian-terrace side-infill variant — typically narrower, cheaper, and PD-friendly.
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The most popular extension type — full pricing by depth, region and spec.
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The full framework for budgeting any UK extension — rear, side, wrap or two-storey.
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