Cost of a Single Storey Extension (2025)
A single storey extension costs £1,500–£2,500 per m². A typical 20m² kitchen extension costs £30,000–£50,000 fully finished.
What does a single storey extension include?
A complete single storey extension includes foundations, structural steel or blockwork walls, roof (flat or pitched), windows and doors, internal plastering, electrics, and plumbing if required. It does not usually include internal decoration or furniture. Always clarify what is and isn't included in any quote.
Do I need planning permission?
Many single storey rear extensions fall under Permitted Development rights — no planning application needed. However, limits apply: the extension cannot exceed 4m (detached house) or 3m (semi/terraced) in depth without the Neighbour Consultation Scheme (where limits increase to 8m/6m with notification). Side extensions almost always need planning permission.
Cost Breakdown
All prices are indicative UK averages for 2025. Actual costs vary by location, specification, and contractor. London and South East rates are typically 20–35% higher.
| Item | Low | High |
|---|---|---|
| Small extension (10–15m²) | £18,000 | £35,000 |
| Medium extension (20–25m²) | £35,000 | £55,000 |
| Large extension (30m²+) | £50,000 | £90,000 |
| Cost per m² (standard) | £1,500 | £2,000 |
| Cost per m² (high spec) | £2,000 | £2,800 |
| Architectural drawings | £800 | £2,500 |
What Affects the Cost?
- ✓Size of the extension in m²
- ✓Roof type — flat roofs are cheaper than pitched
- ✓Ground conditions and foundation type required
- ✓Quality of glazing, bi-fold doors, and finish level
- ✓Location — London adds 20–40% to build costs
- ✓Whether structural engineer calculations are needed
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Post a Job — It's FreeFrequently Asked Questions
How long does a single storey extension take?
A typical 20m² extension takes 10–16 weeks from breaking ground to plastered finish. Add 2–4 weeks for decoration and fitting out.
Do I need an architect?
Not always — many builders will produce working drawings in-house or work with a technician. However, for complex designs or planning applications, an architect adds significant value.
Will an extension add value to my home?
Yes — a well-designed and properly built single storey extension typically adds 10–20% to property value, often recouping most of its cost in areas with high house prices.