Key Takeaway
Compare indicative 2026 two-storey extension build costs, total floor area, VAT and separate project allowances before obtaining contractor quotations.
How do project costs and planning routes compare?
Extensions add to the building, while loft and garage conversions work with existing space; each has different structural and planning considerations.
| Project | Typically needs planning? | Main cost factors | Typical design route |
|---|---|---|---|
| Single-storey extension | Property-specific planning assessment | Footprint, foundations and specification | Survey, layout, planning check and technical drawings |
| Two-storey extension | Planning route assessed for your design | Structure, roof design and specification | Survey, planning check and coordinated structural information |
| Loft conversion | Planning route assessed for your roof design | Roof type, stairs and structural work | Roof and layout assessment, planning check and Building Regulations |
| Garage conversion | Property-specific planning assessment | Insulation, structure and services | Survey, planning check and Building Regulations design |
How to budget for a two-storey extension
A two-storey extension adds ground-floor space and rooms above on a shared footprint. Start with our free extension cost calculator and planning check, then obtain quotations for the actual site and agreed specification.
Have a project in mind? Start with a few details about your home.
Build an initial project budgetIndicative build costs per square metre
Published 2026 UK guidance gives a broad two-storey build range of £1,800–£2,600 per m² + VAT. South East location, access, ground conditions, structural complexity and specification inform your budget.
Measure the combined floor area of both storeys. These worked examples show build allowances + VAT:
| Total new floor area | Indicative build allowance + VAT |
|---|---|
| 30 m² total (15 m² per floor) | £54,000–£78,000 + VAT |
| 50 m² total (25 m² per floor) | £90,000–£130,000 + VAT |
| 70 m² total (35 m² per floor) | £126,000–£182,000 + VAT |
The 50 m² worked build allowance is £90,000–£130,000 + VAT.
What else belongs in the budget?
- Architectural design: Pre-Build from £1,950 + VAT; Premium £2,950 + VAT; Premium Plus £3,950 + VAT. Compare the existing package scope.
- Kitchen, bathrooms, finishes, drainage alterations, utilities and external works.
- A contingency suited to the survey findings.
Planning and structural checks
Some two-storey rear extensions can qualify for permitted development within strict limits; others need planning permission. Check height, depth, boundaries, neighbouring windows, heritage restrictions and the property's planning history. Read the permitted development guide.
Structural calculations are prepared by an independent qualified structural engineer. Foundation loading, the connection to the existing house and structural openings all need consideration. Building regulation drawings are prepared for Building Notice submission; the local authority determines compliance.
Is two-storey always cheaper than single-storey?
Sharing foundations and a roof can reduce the cost per square metre compared with separate additions. Compare written scopes, site conditions, fit-out and the full project cost.
Ready to apply these ideas to your own home?
Check your project costsHampshire and South East cost considerations
Restricted access, ground conditions, structural complexity and specification can increase costs for Hampshire and South East projects. Obtain written quotations for the surveyed site and agreed scope.
Sources and next steps
Indicative range: Mix Design Calc: UK building cost per square metre, 2026. Cross-check: HouseUp: double-storey extension cost and planning guide.
For your address, get a free feasibility study or book a complimentary consultation. Explore Winchester, Basingstoke and our St Albans office.
Two-storey extension cost FAQs
What is the indicative two-storey build cost per square metre?
The indicative 2026 range is £1,800–£2,600 per m² + VAT, measured across the combined floor area of both storeys.
What does the 50 m² worked example cost?
For 50 m² of total new floor area, the worked build allowance is £90,000–£130,000 + VAT.
Is a two-storey extension always cheaper per square metre?
Sharing foundations and a roof can reduce the cost per square metre compared with separate additions. Compare written scopes, site conditions, fit-out and the full project cost.
A useful next step for your home
Share a few project details to explore an initial budget and planning view.
