Planning a house extension is one of the most significant investments you can make in your property. Understanding the true costs involved helps you budget effectively and avoid unwelcome surprises. This comprehensive guide breaks down extension costs across London and Surrey, covering everything from basic builds to high-end finishes.
How Much Does a House Extension Cost?
Extension costs in the UK typically range from £1,500 to £3,500 per square metre, depending on your location, specification, and the complexity of the build. In London and Surrey, expect to pay towards the higher end of this range due to increased labour and material costs.
Average Extension Costs by Type
| Extension Type | Cost Range (London/Surrey) | Typical Size |
|---|---|---|
| Single storey rear | £50,000 – £120,000 | 15-30 sqm |
| Double storey | £80,000 – £200,000 | 30-60 sqm |
| Wrap-around | £70,000 – £180,000 | 25-50 sqm |
| Side return | £40,000 – £80,000 | 10-20 sqm |
| Basement | £200,000 – £500,000+ | 40-100 sqm |
Extension Cost Per Square Metre
The cost per square metre varies significantly based on specification:
- Basic specification: £1,500 – £2,000 per sqm
- Standard specification: £2,000 – £2,800 per sqm
- High specification: £2,800 – £3,500 per sqm
- Premium/bespoke: £3,500 – £5,000+ per sqm
These figures include construction, basic finishes, and standard fixtures. They do not typically include architect fees, planning costs, or high-end fixtures and fittings.
What Affects Extension Costs?
Location
London and Surrey command premium prices compared to other parts of the UK. Central London boroughs like Kensington, Chelsea, and Westminster typically cost 20-40% more than outer London areas. Surrey towns such as Weybridge, Esher, and Cobham also attract higher labour rates due to the affluent housing market.
Structural Complexity
The structural requirements significantly impact costs:
- Simple rear extensions with standard foundations are most economical
- Extensions requiring steel beams for open-plan layouts add £5,000-£15,000
- Party wall agreements with neighbours can add £1,500-£3,000 per wall
- Working near or under trees may require specialist foundations, adding £10,000-£30,000
Groundworks and Foundations
Foundation costs depend on ground conditions and proximity to trees. Standard strip foundations cost around £150-£200 per linear metre. If your site has poor ground conditions or nearby trees, you may need piled foundations, which can add £15,000-£40,000 to your project.
Finishes and Fixtures
The specification level dramatically affects your final cost:
- Kitchen: £10,000 (basic) to £50,000+ (bespoke)
- Bathroom: £5,000 (basic) to £25,000+ (luxury)
- Flooring: £30-£150 per sqm depending on material
- Bifold doors: £4,000-£12,000 depending on size and material
Hidden Costs to Budget For
Many homeowners underestimate the additional costs beyond construction:
Professional Fees
- Architect fees: 8-15% of construction cost
- Structural engineer: £1,000-£3,000
- Planning application: £258 (householder application)
- Building control: £500-£1,500
- Party wall surveyor: £700-£1,500 per party
Site Costs
- Scaffold hire: £1,000-£2,500
- Skip hire: £200-£400 per skip
- Temporary accommodation: Variable
- Storage for furniture: £100-£300 per month
Contingency
Always budget a contingency of 10-15% for unexpected costs. Issues discovered during construction, such as hidden drainage or structural problems, can quickly consume this buffer.
How to Reduce Extension Costs
Design Efficiently
Work with your architect to maximise value:
- Simple rectangular footprints cost less than complex shapes
- Flat roofs are typically cheaper than pitched roofs
- Standardised window and door sizes reduce costs
- Consider permitted development to save planning fees
Choose Materials Wisely
Material choices significantly impact your budget:
- Brick: Traditional, durable, mid-range cost
- Render: Modern look, slightly lower cost than brick
- Timber cladding: Contemporary aesthetic, variable cost
- Glass: Premium cost but adds value and light
Timing Your Project
Construction costs fluctuate with market conditions. Starting your project during quieter periods (autumn/winter) may secure better contractor rates. However, ensure your contractor can manage weather-related delays.
Is an Extension Worth the Investment?
A well-designed extension typically adds more value than it costs. Research suggests that a good quality extension can add 5-10% to your property value, often exceeding the construction cost in high-value areas like London and Surrey.
Beyond financial returns, extensions provide:
- Additional living space tailored to your needs
- Improved natural light and connection to the garden
- Modern, open-plan layouts
- Avoidance of moving costs and stamp duty
Next Steps
Ready to explore your extension options? Contact us for a consultation. We specialise in residential extensions across London and Surrey, from concept design through to completion. Our team will help you understand realistic costs for your specific project and maximise value within your budget.
Learn more about how we work and our design process, or view our portfolio to see examples of completed projects.
We work with homeowners across London and Surrey, including Wandsworth, Wimbledon, Clapham, Battersea, Fulham, Richmond, Kingston, and Guildford.
FAQs
How much does a single storey extension cost in London? A single storey rear extension in London or Surrey typically costs £50,000 to £120,000 for 15 to 30 square metres. The final figure depends on specification, ground conditions and how open plan you want the layout, since steel beams add £5,000 to £15,000.
Do I need planning permission for an extension? Many single storey rear extensions fall under permitted development and do not need a full planning application, though you may still need prior approval. Listed buildings, conservation areas and larger or double storey projects usually require planning permission, so it is worth checking before you design.
How long does a house extension take to build? A typical single storey extension takes 12 to 16 weeks on site once work begins. Larger double storey or wrap around projects can run 4 to 6 months, and the design, planning and tender stages beforehand often add several more months.
What is the most expensive part of an extension? Groundworks and foundations are the most common budget surprise, especially on sites with poor soil or nearby trees, where piled foundations can add £15,000 to £40,000. Kitchens, glazing and bespoke finishes are the biggest controllable costs.
Does an extension add value to my home? A well designed extension usually adds 5 to 10 per cent to property value, which in high value London and Surrey areas often exceeds the build cost. The bigger gain is space tailored to how you live, plus avoiding the cost of moving and stamp duty.
How much should I budget for a contingency? Always set aside 10 to 15 per cent of the construction cost for the unexpected. Hidden drainage, old foundations and structural surprises only appear once work starts, and this buffer keeps the project moving without difficult decisions mid build.
Are architect fees included in the cost per square metre? No. The £1,500 to £3,500 per square metre figures cover construction and standard finishes only. Architect fees run 8 to 15 per cent of construction cost, with structural engineer, planning and building control fees on top.
Why do extensions cost more in London and Surrey? Labour and material rates are higher, and demand for skilled trades is intense. Central London boroughs such as Kensington, Chelsea and Westminster typically run 20 to 40 per cent above outer areas, with affluent Surrey towns like Weybridge, Esher and Cobham close behind.