Short answer: in 2026, a realistic early allowance for a typical house extension in Surrey is around £2,200 to £3,500 per m² for the building work. Complex or high-specification projects can exceed £4,000 per m². A 20m² extension may therefore cost about £44,000 to £70,000 for the core build, before allowing for VAT, design and engineering fees, approvals, specialist finishes and contingency.
Those figures are a starting point, not a quotation. The final cost depends far more on the design, ground conditions, access and specification than it does on floor area alone. Surrey and the wider South East also tend to sit above national averages.
House extension costs per m² in 2026
The table below gives practical planning ranges for homeowners. Always check what is included: a low per-metre figure may cover only a basic shell, while a higher figure may include electrics, heating, plastering, decoration and flooring.
| Extension type | Indicative 2026 build cost | What to expect |
|---|---|---|
| Simple single-storey extension | £2,200–£2,800 per m² | Straightforward shape, standard materials and good access |
| Mid-range single-storey extension | £2,800–£3,500 per m² | Better finishes, larger openings, rooflights or more structural work |
| Complex or high-specification extension | £3,500–£4,500+ per m² | Bespoke glazing, difficult access, premium finishes or major alterations |
| Two-storey extension | £2,000–£3,200 per m² | Often lower per m² because foundations and roof serve both floors, but higher overall |
These are broad South East budgeting ranges. They are not a substitute for drawings, a specification and itemised quotations from suitable contractors.
What does a 20m², 30m² or 40m² extension cost?
Using the typical £2,200–£3,500 per m² range, the core building work may look like this:
| Approximate floor area | Indicative core build | Possible overall project allowance |
|---|---|---|
| 20m² | £44,000–£70,000 | £60,000–£100,000+ |
| 30m² | £66,000–£105,000 | £85,000–£145,000+ |
| 40m² | £88,000–£140,000 | £110,000–£190,000+ |
The overall allowance is deliberately wide. It can include VAT, professional input, statutory fees, fit-out and a contingency, but expensive kitchens, bathrooms, landscaping and unusual structural work can push the total higher.
Costs people commonly leave out
A builder's initial figure does not always represent the complete project cost. Your budget may also need to cover:
- VAT: most work to extend an existing home is charged at the standard 20% rate. HMRC explains the current rules in its VAT guidance for work on houses and flats.
- Design and professional fees: drawings, structural calculations, measured surveys and specialist advice can commonly add around 8–15% of the build cost, depending on the scope.
- Planning and Building Regulations: application, inspection and certification fees vary by council and project.
- Party wall matters: notices and surveyor fees may be needed where work affects a shared structure, boundary or nearby foundations.
- Fit-out: kitchens, bathrooms, flooring, lighting and fitted joinery can materially change the total.
- Contingency: an allowance of around 10–15% is sensible, particularly for older properties or projects involving extensive alterations.
The main factors that change the price
1. Structure and ground conditions
Large openings in the original rear wall, multiple steel beams, deep foundations, poor ground or nearby trees can all add cost. Drainage diversions and build-over agreements may also be required.
2. Access and location
A restricted terraced site costs more to service than a property with direct side access. Labour and contractor demand in Surrey and the South East can also place projects above national averages.
3. Glazing and specification
Large sliding doors, structural glazing, roof lanterns, bespoke cladding, underfloor heating and premium finishes have a disproportionate effect on cost.
4. Changes to the existing house
The new floor area is only part of the job. Reconfiguring rooms, moving services, replacing a kitchen or upgrading the existing property can cost as much as the extension shell.
Do you need planning permission?
Some extensions fall within permitted development rights, while others require a householder planning application. The position depends on the property type, size and location of the work, previous extensions and any local restrictions. The government's householder permitted development guidance explains the national rules.
Even where planning permission is not required, Building Regulations normally still apply. If you are buying a property with extension potential, an independent RICS Level 3 Building Survey can help identify defects, alterations and construction issues before you commit.
Could the Party Wall etc. Act apply?
It may apply if the project involves work to a shared wall, building at or close to a boundary, or excavating near a neighbour's structure. Planning consent does not remove these duties. The government's Party Wall etc. Act guide is a useful starting point.
How to set a realistic extension budget
- Define the floor area and the level of finish you actually want.
- Commission drawings and structural information detailed enough for contractors to price consistently.
- Ask for itemised quotations and confirm whether VAT is included.
- Separate the core build from kitchens, bathrooms, flooring, landscaping and professional fees.
- Keep a 10–15% contingency until the groundworks and structural alterations are complete.
Cheapest is not always best. Compare the same scope, exclusions and quality level across every quotation, and make sure proposed changes to the existing building have been properly considered.
Frequently asked questions
How much does a house extension cost per square metre in 2026?
For early budgeting in Surrey and the wider South East, allow roughly £2,200–£3,500 per m² for typical building work. Complex structures, difficult access, extensive glazing and premium finishes can take the cost above £4,000 per m².
How much might a 20m² extension cost?
The core building work is approximately £44,000–£70,000 at the typical range above. Once VAT, professional fees, approvals, fit-out and contingency are included, an overall allowance of £60,000–£100,000 or more may be more realistic.
Do I need planning permission for a house extension?
Not always. Some projects qualify as permitted development, but the rules depend on the property and proposal. Building Regulations and party wall duties are separate considerations.
Is VAT charged on a house extension?
Most building work on an existing home is subject to the standard 20% VAT rate. Confirm whether every quotation is inclusive or exclusive of VAT.
What makes an extension more expensive?
Structural steelwork, poor ground, drainage changes, limited access, bespoke glazing, premium materials and extensive alterations to the original house are common cost drivers.
Before you buy or alter a property
A home survey does not replace architectural design or a contractor's quotation, but it can reveal defects and construction risks that may affect your plans and budget. Compare our survey services, learn about a RICS Level 2 Home Survey, or contact Surrey Surveyors for independent advice on the property.