By NABC – The National Association of Building Contractors
Thinking About Extending Your Home in 2025?
Building a home extension is one of the most effective ways to increase living space and add value to your property — especially as moving house continues to be expensive across the UK.
However, extension costs in 2025 vary widely depending on several factors, such as the type of extension, property location, building materials, and project complexity.
In this guide, NABC breaks down the average UK costs for single-storey and double-storey extensions, including regional price differences and a detailed cost breakdown by project stage and room type.
By NABC – The National Association of Building Contractors
Thinking About Extending Your Home in 2025?
Building a home extension is one of the most effective ways to increase living space and add value to your property — especially as moving house continues to be expensive across the UK.
However, extension costs in 2025 vary widely depending on several factors, such as the type of extension, property location, building materials, and project complexity.
In this guide, NABC breaks down the average UK costs for single-storey and double-storey extensions, including regional price differences and a detailed cost breakdown by project stage and room type.
How Extension Costs Are Calculated
All cost estimates in this guide are based on the following assumptions:
- The extension has a pitched roof (not flat).
- The structure is made from brick or block, not timber frame.
The room layouts used for our calculations are as follows:
- 20m² single-storey rear extension: dining room, utility room, WC
- 40m² double-storey rear extension: dining room, utility room, WC, two bedrooms, one en-suite bathroom, storage, and small landing
The type of rooms added will influence the total cost. For example, if you’re including a kitchen, you’ll need to factor in the cost of units, fittings, and appliances.
💡 Note: Figures only cover the cost of new rooms — not the remodelling of existing spaces.
Build Specification Levels
Extension costs depend heavily on the quality of materials and finish. To help homeowners budget accurately, we use three build specification levels:
- Low specification – Basic finish (e.g. Leyland, Everest, Wickes)
- Mid specification – Standard finish (e.g. Dulux, Hamilton, Slim Line)
- High specification – Premium finish (e.g. Farrow & Ball, Fine Line, Siemens)
Each project can also vary depending on the contractor’s size and experience:
- Low benchmark – Small contractors, lower cost, limited specialisation.
- Mid benchmark – Medium-sized contractors, balanced cost and quality.
- High benchmark – Large firms, higher cost base, suited for complex projects.
Single-Storey Rear Extension Costs (Standard Specification)
| Region | Low Benchmark | Mid Benchmark | High Benchmark | Low £/m² | Mid £/m² | High £/m² |
| East Anglia | £79,067 | £86,255 | £93,443 | £2,636 | £2,875 | £3,115 |
| London | £83,258 | £93,492 | £103,834 | £2,775 | £3,116 | £3,461 |
| Midlands | £76,740 | £83,716 | £90,693 | £2,558 | £2,791 | £3,023 |
| North England | £74,934 | £81,746 | £88,558 | £2,498 | £2,725 | £2,952 |
| Northern Ireland | £77,293 | £84,275 | £91,298 | £2,575 | £2,809 | £3,043 |
| Scotland | £77,334 | £84,364 | £91,395 | £2,578 | £2,812 | £3,046 |
| South East | £83,142 | £92,968 | £102,038 | £2,771 | £3,099 | £3,401 |
| South West | £77,518 | £89,564 | £98,302 | £2,670 | £2,985 | £3,277 |
| Wales | £74,938 | £83,795 | £89,986 | £2,498 | £2,793 | £3,000 |
| Average | £78,247 | £86,686 | £94,394 | £2,618 | £2,890 | £3,146 |
All costs exclude VAT.
Key Insights
- It costs around 9% more to build an extension in London than in other UK regions.
- Minimum total cost: £74,934
- Maximum total cost: £103,834
- UK average: £86,443 or £2,885 per square metre
Cost Breakdown by Project Stage – London (Mid Benchmark)
| Project Stage | Cost (£) | % of Total |
| External walls & lining | £14,024 | 15.0% |
| Foundations & substructure | £13,159 | 14.1% |
| Roof structure & coverings | £11,540 | 12.3% |
| Steel & structural supports | £8,133 | 8.7% |
| Floor prep & finishes | £5,966 | 6.4% |
| Kitchen units & appliances | £4,457 | 4.8% |
| Heating & cooling | £4,198 | 4.5% |
| Windows & doors | £3,984 | 4.3% |
| Decoration | £3,927 | 4.2% |
| Preliminaries | £3,564 | 3.8% |
| Floor structure & lining | £3,555 | 3.8% |
| Wired appliances | £3,239 | 3.5% |
| Internal walls & lining | £2,711 | 2.9% |
| Drainage & pipework | £2,427 | 2.6% |
| Wall prep & finishes | £2,163 | 2.3% |
| Internal doors & frames | £1,517 | 1.6% |
| Plumbing fixtures | £1,373 | 1.5% |
| Demolition | £1,346 | 1.4% |
| Wiring & fuseboards | £634 | 0.7% |
| Ceilings (structure & finishes) | £1,148 | 1.2% |
| Fixtures & fittings | £426 | 0.5% |
Total: £93,492 (100%)
Cost Breakdown by Room – London (Mid Benchmark)
| Room Type | Cost (£) | % of Total |
| Ground Floor Rear Extension (pitched roof) | £60,362 | 64.6% |
| Utility Room | £13,821 | 14.8% |
| Dining Room | £10,902 | 11.7% |
| WC | £7,912 | 8.5% |
| Preliminaries | £496 | 0.5% |
Double-Storey Rear Extension Costs (Standard Specification)
| Region | Low Benchmark | Mid Benchmark | High Benchmark | Low £/m² | Mid £/m² | High £/m² |
| East Anglia | £131,357 | £143,298 | £155,240 | £2,189 | £2,388 | £2,587 |
| London | £138,434 | £155,344 | £173,574 | £2,307 | £2,589 | £2,893 |
| Midlands | £127,393 | £138,974 | £150,555 | £2,123 | £2,316 | £2,509 |
| North England | £124,191 | £135,481 | £146,771 | £2,070 | £2,258 | £2,446 |
| Northern Ireland | £128,347 | £139,658 | £151,296 | £2,134 | £2,328 | £2,522 |
| Scotland | £128,673 | £140,371 | £152,069 | £2,145 | £2,340 | £2,534 |
| South East | £138,130 | £154,455 | £169,524 | £2,302 | £2,574 | £2,825 |
| South West | £128,668 | £148,454 | £162,937 | £2,213 | £2,474 | £2,716 |
| Wales | £124,572 | £139,295 | £149,616 | £2,076 | £2,322 | £2,494 |
| Average | £129,974 | £143,926 | £156,842 | £2,173 | £2,399 | £2,614 |
All costs exclude VAT.
Key Insights
- Typical price range: £124,191 – £173,574
- Average total cost: £143,581
- Average cost per square metre: £2,395
Final Thoughts
Extension costs in 2025 continue to rise modestly due to material and labour inflation, but extending remains a cost-effective way to add living space and boost property value.
Before beginning any project, always:
- Obtain multiple quotes from vetted contractors.
- Ensure your builder is NABC accredited builders for quality and compliance.
- Consider planning permissions and Building Regulations.
For more accurate, region-specific pricing, or to find trusted extension specialists near you, visit the NABC contractor directory.