Single Storey Extension Cost Calculator UK
Get a 2026 single storey extension cost estimate in 30 seconds. Spec level, floor area, kitchen or bathroom fit-out, glazing and region — real UK supply-and-fit rates including VAT. No email required.
Estimate Your Single Storey Extension Cost in 30 Seconds
5 steps. Covers foundations, shell, roof, glazing, plastering, decoration and VAT. Professional fees (10–15%) and contingency are flagged separately in the result notes.
How This Calculator Works
Our single storey extension calculator combines spec level, floor area, fit-out and region with real 2026 UK build pricing from vetted builders across our 519-town database.
If you are still comparing project types, our general extension cost calculator lets you price other layouts side by side, while homeowners short on garden space often weigh an extension against going upwards — our loft conversion cost guide sets out how that alternative compares before you settle on a single-storey build.
Build Rates (per m², complete)
- Basic spec — £1,700–£2,000/m²
- Standard spec — £1,900–£2,600/m²
- Premium / glass-heavy — £2,800–£3,500/m²
- Kitchen fit-out +£15,000 · bathroom/utility +£6,000
- Bifold/sliding doors +£5,000 · structural glazing feature +£12,000
Rates include foundations, shell, roof, glazing, plastering, decoration, labour and VAT.
Assumptions & Adjustments
- Add 10–15% for professional fees (architect, engineer, surveys) and 10–15% contingency
- Permitted development: rear extensions up to 3m (semi/terraced) or 4m (detached) usually need no planning
- Party wall agreements £700–£1,500 per affected neighbour where applicable
- Region multipliers: London +27% … Northern Ireland −12% (Midlands baseline)
- Estimate only — a firm quote always follows a site survey
Building regulations always apply to habitable extensions — your builder handles sign-off.
Typical Single Storey Extension Costs (2026)
Complete build incl. foundations, shell, roof, glazing, decoration & VAT, Midlands baseline. Professional fees (10–15%) and contingency extra.
| Scenario | Size | Cost range |
|---|---|---|
| Standard spec | 15 m² (3×5m) | £28,500–£39,000 |
| Basic spec | 20 m² | £34,000–£40,000 |
| Standard spec | 20 m² (4×5m) | £38,000–£52,000 |
| Standard spec + kitchen fit-out | 25 m² | £62,500–£80,000 |
| Premium / glass-heavy | 30 m² | £84,000–£105,000 |
Want the full method and per-material detail? Read the complete 2026 roof replacement cost guide.
What a Single Storey Extension Price Actually Includes
The per-square-metre rates in this calculator cover the full building shell: foundations, brick or block walls, the roof structure and covering, standard glazing, first and second fix, plastering, decoration, labour and VAT. What they do not automatically include is the fit-out of specialist rooms. A kitchen extension adds roughly £10,000-£25,000 for units, worktops, appliances and plumbing on top of the shell, while a bathroom or utility adds around £4,000-£8,000.
Because that kitchen fit-out is such a large slice of the total, it is worth pricing it in its own right rather than treating it as an afterthought — our kitchen cost calculator breaks the units, worktop and appliance spend down by size and finish so you can drop a realistic fit-out figure into your extension budget.
Two further costs sit outside the headline build figure and catch homeowners out. First, professional fees - an architect or technologist, a structural engineer for the steel beam calculations, and building control - typically run 10-15% of the build cost, and a contingency of another 10-15% is sensible for older properties where ground conditions are unknown. Second, if you build against or near a shared boundary you will usually need a party wall agreement, which costs about £700-£1,500 per affected neighbour. Deep or reinforced foundations on poor ground, sloping sites or near mature trees can also add several thousand pounds that no calculator can predict before a site survey.
Planning Permission and Building Regulations
Many single storey extensions are built under permitted development rather than a full planning application. A rear extension can usually project up to 3m beyond the original rear wall for a semi-detached or terraced house, or 4m for a detached house, within a 4m height limit. Larger single storey rear extensions - up to 6m for an attached home or 8m for a detached one - are allowed under the Larger Home Extension prior-approval route, where the council notifies your neighbours before agreeing the work. Conservation areas, listed buildings and homes that have had their permitted development rights removed are excluded, so confirm your position on the Planning Portal extensions guidance before you commit.
Building regulations, by contrast, apply to almost every habitable single storey extension regardless of planning - they govern structure, foundations, insulation, fire safety, ventilation and drainage, and completion is confirmed by a building control sign-off you will need when you sell. For a room-by-room view of where the rules bite, read our rear extension planning guide. Treat any calculator figure as a budget anchor and always let a firm quote follow a full site survey.
Frequently Asked Questions
How much does a single storey extension cost in 2026?
Do I need planning permission for a single storey extension?
How much does a kitchen extension add on top of the build?
What is not included in the calculator estimate?
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