How Much Does a Home Extension Cost in the UK?
Real 2026 prices per square metre by extension type and spec, a worked example, and exactly how to calculate a number for your own house — not the one a builder mentions over a cup of tea and never puts in writing.
Home extension cost by spec level
Spec level moves the price more than most people expect — the same size extension at a different finish can differ by £1,000+ per square metre.
Basic
- Simple form, flat roof
- Standard windows and doors
- Builder-grade fittings
Standard
- Where most family extensions land
- Pitched or vaulted roof options
- Bifold or sliding doors
Premium
- Structural glazing, roof lanterns
- Underfloor heating throughout
- High-end joinery and finishes
Cost by extension type
The shape of your extension matters as much as its size:
| Extension type | Typical cost per m² |
|---|---|
| Single-storey rear | £1,800–£3,000 |
| Double-storey | £1,500–£2,400 |
| Side return | £2,500–£3,500 |
| Wraparound | £2,400–£3,300 |
Double-storey extensions cost less per square metre because one foundation and one roof serve two floors of new space — the saving is in shared structure, not in doing less work. Side returns and wraparounds cost more per m² because of the awkward, often narrow shapes involved and the structural work needed where they meet the existing house.
How to calculate a home extension cost
You don't need a quantity surveyor to get a realistic ballpark — the same method every cost guide uses works fine for early planning:
-
1. Work out your floor area
Measure length × width in metres to get your extension's floor area in m². This is the number everything else multiplies from, so measure twice.
-
2. Pick your extension type
Single-storey, double-storey, side return, or wraparound — use the cost-by-type table above to find your starting £/m² range.
-
3. Choose a spec level
Basic, standard, or premium (see the cards above) narrows your £/m² range to a single realistic figure for your finish level.
-
4. Multiply area × rate
Floor area (m²) × your chosen £/m² rate gives your build cost — labour and materials only, before anything else.
-
5. Add VAT, fees, and contingency
Add 20% VAT, £3,000–£10,000 for professional fees, and 10–15% contingency on top of the build cost. This step is where most DIY estimates fall short of the real final number.
A worked example: 20m² single-storey extension
A realistic breakdown for a standard-spec single-storey rear extension, a common size and type for a Leicestershire semi:
| Item | Estimated cost |
|---|---|
| Build cost (20m² @ £2,200/m², standard spec) | £44,000 |
| VAT (20%) | £8,800 |
| Professional fees (architect, structural engineer, Building Regs) | £5,000 |
| Planning application fee (if required) | £548 |
| 10% contingency (on build cost) | £4,400 |
| Total | £62,748 |
What affects the price
Property type
1930s semis — common across Leicestershire — are usually well suited to a straightforward single-storey rear extension, often within Permitted Development limits. Victorian and Edwardian terraces more often need a side return, which costs more per m² and may involve a party wall agreement with a neighbour — the most polite legal document in Britain, and also one of the slowest. Detached houses generally have the most flexibility and the fewest boundary-related restrictions.
- Location — London and the South East run 25–40% above these figures; the Midlands and North are typically at or below the UK average
- Ground conditions — poor soil or nearby trees can require more expensive foundations
- Planning permission — many single-storey rear extensions fall under Permitted Development, but size and boundary limits apply; larger or two-storey extensions usually need full planning permission
- Party wall agreements — required for most side-return extensions and some double-storey builds, adding time and sometimes a surveyor's fee
Local considerations for Leicestershire homes
Extension costs in Leicestershire and the East Midlands generally sit at or below the UK national average, unlike the 25–40% premium seen in London and the South East. The more relevant local factor is property type: Hinckley's mix of 1930s semis and Victorian terraces means the right extension type (and whether Permitted Development applies) varies more house to house than the regional cost band does.
How to avoid overspending
- Get at least 3 quotes from local, vetted builders — costs can vary 20–30% for the same spec
- Confirm whether you need planning permission or a Lawful Development Certificate before you commit to a design
- Budget professional fees and VAT from the start, not as an afterthought once the build quote lands
- Add the 10–15% contingency as standard, especially on older properties where ground conditions are less predictable
None of this is complicated. Most extensions go over budget because someone skipped step 5 above, not because the building work itself was hard.
See our full Home Extensions page for Renofox's own extension service across Hinckley and Leicestershire. Also considering a loft conversion instead of, or as well as, an extension? Or need a bathroom renovation as part of the same project? We cover all three.
Frequently asked questions
How much does a home extension cost in the UK?
Typically £1,500 to £3,500 per square metre depending on the type and spec. A standard single-storey rear extension of around 20m² usually costs £40,000 to £56,000 before VAT and professional fees.
How do I calculate a home extension cost?
Multiply your extension's floor area in square metres by the per-m² rate for your extension type and spec level, then add 20% VAT, professional fees of roughly £3,000 to £10,000, and a 10 to 15% contingency on top of the build cost.
Is a double-storey extension cheaper per square metre than a single-storey one?
Yes. A double-storey extension typically costs £1,500 to £2,400 per square metre, compared with £1,800 to £3,000 for a single-storey extension, because the foundations and roof are shared across two floors. The total project cost is still higher, since you're building more floor area.
Do I need planning permission for a home extension?
Many single-storey rear extensions fall under Permitted Development and don't need planning permission, but there are size and boundary limits. Larger extensions, side returns near a boundary, and most two-storey extensions usually do need planning permission or at least a Lawful Development Certificate to confirm Permitted Development rights.
What's included in the per-square-metre extension price?
The per-m² build rate covers labour and materials only. VAT, architect and structural engineer fees, Building Regulations fees, and any planning application fee are usually on top, not included in the headline £/m² figure.
How much do professional fees add to an extension cost?
Budget roughly £3,000 to £10,000 for architect drawings, a structural engineer, and Building Regulations approval, depending on the complexity of the extension.
Get a quote for your extension
National averages are a starting point, not a quote. Get a free, no-obligation quote based on your actual house, in your actual area.
Get a free quote