A clear and realistic 2026 cost guide for house extensions in Surrey and SW London. We explain average extension prices, what factors affect your final build cost, and practical ways to avoid budget overruns.
House Extension Costs in Surrey & SW London (2026)
One of the first questions homeowners ask when planning an extension is “how much will it really cost?”. In Surrey and South West London, extension prices in 2026 continue to reflect high demand, skilled labour shortages, and rising material standards. Understanding realistic budgets early helps you design within your means and avoid expensive changes later.
- Costs vary significantly by extension type and size
- Location and access strongly influence pricing
- Early design decisions have the biggest impact on budget
| Typical House Extension Costs (2026) |
|---|
|
Single-storey rear extension: £2,200 – £3,200 per m² Side return extension: £2,500 – £3,500 per m² Two-storey extension: £2,300 – £3,400 per m² Wraparound extension: £2,800 – £3,800 per m² |
What Drives the Cost of a House Extension?
Extension costs are influenced by more than just floor area. The complexity of the build, structural requirements, and quality of finishes all play a major role in the final price. Homes in Surrey and SW London often require higher-spec materials and careful structural solutions, which increases costs compared to national averages.
- Structural steel and groundworks complexity
- Access restrictions and site logistics
- Specification of glazing, kitchens, and finishes
| Main Cost Influencers |
|---|
|
Ground conditions and foundations Structural alterations to existing house Energy efficiency and insulation standards Bespoke design features and finishes |
Hidden Costs Homeowners Often Overlook
Many budget overruns occur because early estimates don’t include all project costs. Planning, technical compliance, and finishing items can add up quickly if not allowed for from the outset.
- Professional fees and surveys
- Building control and inspection fees
- External works and landscaping
| Commonly Missed Budget Items |
|---|
|
Architectural and structural design fees Party wall agreements Utility upgrades and drainage works Decoration and final fit-out |
How to Avoid Cost Surprises on Your Extension
The most successful extension projects are those that are fully designed, specified, and priced before work begins. Clear documentation and professional oversight reduce variations and keep spending under control.
- Finalise design and layout before construction
- Use a detailed specification, not estimates
- Allow a sensible contingency budget
| Best Practice Cost Control |
|---|
|
Fixed-price contracts where possible Transparent breakdown of costs Early structural and ground investigations Regular budget tracking during the build |
House Extension Cost FAQs
Answers to common cost and budgeting questions homeowners ask before extending in Surrey and SW London.
How much does a house extension cost in Surrey in 2026?
Most house extensions in Surrey cost between £2,200 and £3,800 per square metre in 2026, depending on size, complexity, and finish.
Are extensions more expensive in SW London than Surrey?
Yes, extensions in SW London are often slightly more expensive due to access constraints, parking restrictions, and higher labour costs.
What type of extension offers the best value for money?
Single-storey rear and side return extensions typically offer the best cost-to-space ratio and strong resale value.
Should I budget a contingency for my extension?
Yes. A contingency of 8–12% is recommended to cover unforeseen ground conditions, structural changes, or specification upgrades.
Do extension costs include VAT?
Most extension works are subject to VAT. Always confirm whether quotes are inclusive or exclusive of VAT.
Can poor design increase extension costs?
Yes. Incomplete designs and late changes are one of the biggest causes of cost overruns. Proper design coordination saves money overall.
