The short answer: £2,000–£3,500 per square metre in Cambridge. But that range hides enormous variation. A 20m² kitchen extension could cost £40,000 or £80,000 depending on what you choose. This guide explains what you're actually paying for — so you can estimate before you quote.
The Quick Answer
| Extension Type | Cost Per m² | Example (20m²) |
|---|---|---|
| Single-storey, basic | £1,800–£2,200 | £36,000–£44,000 |
| Single-storey, mid-range | £2,200–£2,800 | £44,000–£56,000 |
| Single-storey, premium | £2,800–£4,000+ | £56,000–£80,000+ |
| Double-storey | £2,500–£3,500 | £50,000–£70,000 (per floor) |
These are Cambridge/South East prices. Northern England and Wales are typically 15–25% lower.
Why Is the Range So Wide?
The same 20m² extension could cost £40,000 or £80,000. Here's what makes the difference:
1. Specification Level (Biggest Factor)
The difference between "basic" and "premium" is often £1,000+ per m². What counts as premium?
- Windows and doors: uPVC (£500–£800 per opening) vs aluminium/timber (£1,500–£3,000)
- Flooring: Carpet/vinyl (£30–£50/m²) vs engineered oak (£100–£150/m²)
- Heating: Radiator extension (£500–£1,000) vs underfloor heating (£2,000–£4,000)
- Kitchen: Budget (£5,000–£10,000) vs premium (£15,000–£30,000)
2. Structural Choices
- Roof: Flat roof is cheapest (£80–£120/m²); pitched adds 30–50%
- Glazing: Standard windows vs bifold doors (£4,000–£8,000 for 3m)
- Foundations: Standard strip vs piled (if poor ground conditions)
3. Site Conditions
Two identical extensions on different sites can have very different costs:
- Access: Poor access (no side passage) adds 20–30%
- Ground: Clay soil, high water table, nearby trees increase foundation costs
- Existing structure: Demolition, party wall issues, drainage relocation
How to Estimate Before You Quote
You don't need a builder to get a rough idea. Here's a quick method:
- Measure your planned extension (length × width = m²)
- Choose your spec level: basic (£2,000/m²), mid-range (£2,500/m²), premium (£3,500/m²)
- Multiply: m² × cost per m²
- Add 10% for contingency
Example: 5m × 4m extension = 20m². Mid-range spec = £2,500/m². Base estimate = £50,000 + £5,000 contingency = £55,000.
Hidden Costs People Forget
- Planning fees: £206 for householder application (or £0 for permitted development)
- Building regs: £500–£1,500 depending on complexity
- Architect: £1,000 (basic drawings) to £5,000+ (full service)
- Structural engineer: £500–£2,000
- Party wall surveyor: £500–£1,500 if you share a wall
- VAT: Most prices quoted don't include 20% VAT
Get an Accurate Quote
Online calculators and guides give you a range. The only way to know what your extension will actually cost is to get a detailed quote based on your specific requirements.