The best way to understand extension costs is to see what other people actually paid. Here are real projects we've completed in Cambridge and Cambridgeshire, with before and after photos, itemised costs, and what affected the final price.
Example 1: Single-Storey Rear Extension — £52,000
Project Overview
- Location: Cherry Hinton, Cambridge
- Type: Rear extension, kitchen-diner
- Size: 25m²
- Cost per m²: £2,080
- Duration: 10 weeks
What Was Included
A 25m² single-storey extension to the rear of a 1930s semi-detached house, replacing a small conservatory. The extension created an open-plan kitchen and dining space with bifold doors to the garden.
Cost Breakdown
| Item | Cost |
|---|---|
| Foundations (strip foundations, 1.2m deep) | £6,200 |
| Walls (brick and block cavity) | £9,500 |
| Roof (flat EPDM with skylights) | £5,800 |
| Floor (insulated concrete slab) | £3,200 |
| Bifold doors (3m aluminium) | £4,800 |
| Windows (2no. aluminium) | £2,400 |
| Electrics and lighting | £3,500 |
| Underfloor heating | £2,800 |
| Plastering and finishing | £3,600 |
| Kitchen (units, worktops, appliances) | £8,200 |
| Professional fees (architect, structural engineer) | £2,000 |
Total: £52,000
What Affected the Price
- Good access: Side passage meant materials could be brought in easily
- Demolition required: Existing conservatory added £2,500
- Premium finish: Bifold doors and underfloor heating added £7,600
Example 2: Double-Storey Extension — £95,000
Project Overview
- Location: Trumpington, Cambridge
- Type: Side and rear extension
- Size: 35m² ground floor + 35m² first floor = 70m² total
- Cost per m²: £1,360 (total) / £2,700 (per floor)
- Duration: 16 weeks
What Was Included
A side-return and rear extension adding a new kitchen-diner on the ground floor and a master bedroom with en-suite above. This wrapped around the side of a detached house.
Cost Breakdown
| Item | Cost |
|---|---|
| Foundations | £8,500 |
| Structure (walls, floors, roof) | £28,000 |
| Windows and doors | £6,200 |
| Electrics (rewire, smart home) | £5,500 |
| Plumbing and heating (new en-suite) | £4,800 |
| Internal fit-out | £12,000 |
| Kitchen (high-end units, appliances) | £12,500 |
| Bathroom (en-suite) | £7,500 |
| Professional fees | £5,000 |
| Planning and building regs | £5,000 |
Total: £95,000
Key Takeaways
Double-storey extensions cost less per square metre because the expensive parts — foundations and roof — don't double. You're adding floor area without doubling the structural cost.
Example 3: Wrap-Around Extension — £78,000
Project Overview
- Location: Histon, Cambridgeshire
- Type: Wrap-around (side infill + rear)
- Size: 30m²
- Cost per m²: £2,600
- Duration: 12 weeks
Cost Breakdown
| Item | Cost |
|---|---|
| Foundations and groundworks | £12,000 |
| Structure | £22,000 |
| Roof (pitched, tiled) | £9,500 |
| Glazing (Velux windows, bi-folds) | £8,200 |
| Internal fit-out | £15,000 |
| Professional fees | £3,000 |
| Planning and building regs | £3,300 |
| Contingency used | £5,000 |
Total: £78,000
What Affected the Price
- Roof choice: Pitched roof added £3,500 vs flat but increased value
- Glazing: 5no. rooflights + bifold doors = significant cost
- Contingency: Used for drainage rerouting discovered during groundworks
What Affects Extension Cost Most
Across all three examples, these factors had the biggest impact:
- Specification level: The difference between standard and premium fit-out can be £1,000/m²
- Access: Poor access (no side passage, narrow streets) adds 20-30%
- Ground conditions: Clay soil and drainage issues increase foundation costs
- Glazing: Bifold doors, rooflights, and large windows are expensive but transform the space
- Planning: Full planning adds time and fees; permitted development is cheaper
Get a Quote for Your Extension
Every extension is different. These examples show what's possible, but the only way to know what your project will cost is to get a detailed quote based on your specific requirements.
Frequently Asked Questions
How accurate are online extension cost calculators?
Online calculators give a rough estimate but can be 30-50% off. They don't account for access, ground conditions, or specification level. Use them as a starting point, then get real quotes.
What's the cheapest way to extend a house?
A single-storey rear extension with standard specification and good access is typically cheapest. Avoid basements, large glazing, and complex roof designs if budget is tight.
How much does a kitchen extension cost?
A 20-25m² kitchen extension with mid-range kitchen costs £45,000-£70,000 in Cambridge. Premium kitchens can push this to £100,000+.