Cost of House Extension London (2026 Guide)
How Much Does a House Extension Cost in London in 2026?
If you're planning a house extension in London, one of the first questions is always:
“How much will it really cost?”
In 2026, extension costs in London continue to rise due to labour shortages, stricter building regulations, and increasing material prices. This guide provides realistic, contractor-level pricing — based on actual projects, not outdated online estimates.
Average Cost of House Extensions in London (2026)
In London, house extension costs typically fall into three tiers:
- Basic build (structure only): from £1,800 per m²
- Standard extension (most projects): £2,500 – £3,200 per m²
- High-spec finish: £3,500+ per m²
???? Most homeowners fall within the £2,500 – £3,200 per m² range depending on design and finishes.
Extension Cost by Type
Rear Extension Cost London
- Most common option for terraced and semi-detached homes
- Typical size: 20–50m²
£60,000 – £150,000+
Side Return Extension Cost
- Popular in Victorian and Edwardian properties
- Smaller footprint but more structural complexity
£45,000 – £100,000+
Wraparound Extension Cost
- Combines rear + side extension
- Maximises space and layout flexibility
£100,000 – £200,000+
Double Storey Extension Cost
- Adds space on both floors
- Better value per m² compared to single-storey
£2,200 – £3,000 per m²
⚠️ Why Some Quotes Seem Much Cheaper
You may come across quotes that are significantly lower than the ranges above.
In many cases, these quotes exclude essential elements such as:
- Structural steel (RSJs)
- Foundations and groundworks
- Drainage works
- Internal finishes
- Electrical and plumbing
These missing items can easily add £20,000 – £50,000+ during construction.
We believe in transparent pricing from the start, so you can plan your budget with confidence.
Key Factors That Affect Cost
1. Labour Costs in London
Skilled labour is one of the biggest cost drivers:
- Electricians: £250–£400/day
- Plumbers: £200–£350/day
2. Structural Complexity
Costs increase significantly if your project involves:
- Load-bearing wall removal
- Steel beam installation
- Open-plan layouts
3. Materials
Material costs remain volatile in 2026:
- Timber
- Insulation
- Steel
4. Access & Location
Projects in Central or West London often cost more due to:
- Limited site access
- Parking restrictions
- Manual handling
5. Interior Finishes
Final cost depends heavily on specification:
- Kitchen: £5,000 – £30,000+
- Bathrooms
- Flooring
- Lighting
Real Project Example (London)
Project: 50m² rear extension + full internal refurbishment
| Item | Cost |
|---|---|
| Structure & shell | £95,000 |
| Steelwork | £12,000 |
| Roof & skylights | £10,000 |
| Interior & kitchen | £35,000 |
| Total | ~£150,000 |
Can You Reduce the Cost?
Yes — with the right planning:
- Keep the design simple
- Avoid unnecessary structural changes
- Finalise layout early
- Choose materials strategically
- Consider phased construction
If you're working with a tighter budget, a good builder can help adjust the design without compromising quality.
Fixed Price vs Day Rate
Most London extensions are done on a fixed-price basis, which provides:
✔ Cost certainty
✔ Clear scope
✔ Easier budgeting
Some projects may use hybrid or day-rate approaches for flexibility, but these require careful management.
Common Mistakes to Avoid
- Choosing the cheapest quote
- Underestimating the total budget
- Making design changes mid-build
- Not planning services (electrical / plumbing) early
These are the most common causes of delays and cost overruns.
Final Advice
A realistic budget for a house extension in London in 2026 is:
£2,500 – £3,200 per m²
Anything significantly lower is usually missing key elements or not fully scoped.
Get a Transparent Quote
Every property is different.
At Canghai Construction Ltd, we provide:
- Clear, itemised quotations
- Practical construction advice
- Honest guidance based on real experience
If you're planning an extension, feel free to get in touch for a tailored quote.