A single-storey extension in the UK typically costs £1,500–£2,500 per square metre, meaning a standard 20m² extension usually falls between £30,000–£50,000 including a basic fit-out. Costs rise with higher-spec finishes, structural complexity, or if the extension includes a kitchen or bathroom.
Average Extension Costs by Size
| Extension Size | Basic Finish | Mid-Range Finish | High-End Finish |
|---|---|---|---|
| 10m² | £15,000–£25,000 | £25,000–£35,000 | £35,000+ |
| 20m² | £30,000–£50,000 | £50,000–£70,000 | £70,000+ |
| 30m² | £45,000–£75,000 | £75,000–£105,000 | £105,000+ |
These figures are for the build shell plus a basic internal finish. Fitted kitchens, bespoke glazing, underfloor heating, and bi-fold doors are typically priced separately and can add £10,000–£30,000+ depending on spec.
What Affects the Price?
Foundations and ground conditions. Difficult access, sloping sites, or poor soil conditions requiring deeper foundations can add significant cost before any walls go up.
Roof type. A flat roof is generally the cheapest option; a pitched or hipped roof matching the existing house costs more but often adds more resale value and blends better architecturally.
Glazing. Large bi-fold or sliding doors look great but are one of the biggest cost variables in a modern extension — expect £3,000–£8,000+ depending on size and quality.
Services. Extending electrics, plumbing, heating, and drainage into the new space adds cost, especially if the extension includes a kitchen, utility room, or bathroom.
Do I Need Planning Permission?
Many single-storey extensions fall under Permitted Development Rights and don't need full planning permission — but there are limits (typically extending up to 6–8m from the original rear wall, depending on whether it's a detached or terraced/semi-detached house, and height restrictions). It's always worth checking with your local planning authority or a qualified architect before starting, since building without required permission can mean being forced to alter or remove the work later.
Regardless of planning permission, Building Regulations approval is always required for an extension — this covers structural safety, insulation, fire safety, and drainage, and is separate from planning permission.
Typical Build Timeline
A single-storey extension typically takes 8–14 weeks from breaking ground to a fully finished space, depending on size and weather delays during groundworks. Add extra time if you're also fitting a kitchen or bathroom within the new space.
Getting an Accurate Quote
A proper extension quote should clearly separate:
- Groundworks and foundations
- Structural shell (walls, roof)
- First-fix plumbing and electrics
- Windows and doors (glazing is often a major cost variable)
- Plastering, flooring, and second-fix finishing
- Whether Building Control fees and architect/structural engineer fees are included
Compare Extension Builder Quotes
Extension costs vary substantially between builders for the same brief, so getting at least 2–3 quotes is essential before committing. Post your job for free on Compare My Builder and get matched with verified local builders experienced in extensions.
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