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.
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.
Real Project: 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.
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Common Questions
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