How Much Does a Rear Extension Cost in 2026?
The honest answer: it depends on the size, what is inside it, and where you live. But vague answers are not helpful, so this guide gives you real numbers based on typical UK projects in 2026.
A straightforward single-storey rear extension in England costs between £1,400 and £3,500 per square metre for the build itself. A 20m² kitchen-diner extension at mid-range specification typically lands between £40,000 and £70,000 including professional fees.
Last updated: April 2026
Build costs by extension type
These are build costs only — the construction work itself. Professional fees, planning, building regulations, and other costs are listed separately below.
Single-storey rear extension
| Specification level | Cost per m² | 15m² extension | 20m² extension | 30m² extension |
|---|---|---|---|---|
| Basic (utility, simple living room, standard finishes) | £1,400–£2,000 | £21,000–£30,000 | £28,000–£40,000 | £42,000–£60,000 |
| Mid-range (kitchen-diner, good finishes, bi-fold doors) | £2,000–£2,800 | £30,000–£42,000 | £40,000–£56,000 | £60,000–£84,000 |
| High-spec (bespoke kitchen, underfloor heating, vaulted ceiling, premium materials) | £2,800–£3,500 | £42,000–£52,500 | £56,000–£70,000 | £84,000–£105,000 |
Two-storey rear extension
Two-storey extensions cost roughly 50–70% more than single-storey — not double, because you share the roof and foundations across two floors.
| Specification level | Cost per m² (total floor area) | 30m² total (15m² per floor) | 40m² total (20m² per floor) |
|---|---|---|---|
| Mid-range | £1,800–£2,500 | £54,000–£75,000 | £72,000–£100,000 |
| High-spec | £2,500–£3,200 | £75,000–£96,000 | £100,000–£128,000 |
What is included in the build cost?
A typical build cost per m² includes:
- Foundations and groundworks
- External walls, insulation, and render or brickwork
- Roof structure and covering
- Internal walls and plastering
- Windows and doors (including bi-folds if specified)
- Floor screeds and insulation
- First fix electrics and plumbing
- Second fix electrics, plumbing, and heating
- Decorating (paint, not wallpaper)
- Basic flooring allowance
A typical build cost does not include:
- Kitchen units and appliances
- Bathroom fittings (if the extension includes a WC)
- Underfloor heating (add £2,000–£4,000)
- Specialist flooring (engineered oak, porcelain tiles)
- Landscaping and reinstatement of the garden
- Furniture and furnishings
Professional fees
On top of the build cost, budget for these professional fees:
| Fee | Typical range | Notes |
|---|---|---|
| Architect or designer | £1,500–£5,000 | Design, drawings, and potentially project management. A chartered architect is more expensive than an architectural technologist or designer. |
| Structural engineer | £300–£800 | Structural calculations for foundations, steelwork, and lintels. Almost always required. |
| Planning application (if needed) | £548 | Householder planning fee. Not needed if building under permitted development. |
| Prior approval (if applicable) | £249 | For larger single-storey extensions using the prior approval process. |
| Lawful Development Certificate | £274 | Recommended if building under PD. Official confirmation the extension is lawful. |
| Building regulations | £400–£900 | Covers plan check and inspections. Required for every extension. |
| Party wall surveyor | £700–£1,500 per neighbour | Only if the Party Wall Act applies (building on or near a boundary). |
Total professional fees: typically £3,000–£8,000 on top of the build cost.
Regional variations
Build costs vary significantly across England. London and the South East are the most expensive. The Midlands and the North are generally more affordable.
| Region | Approximate adjustment |
|---|---|
| London (inner) | +30–50% |
| London (outer) and South East | +15–30% |
| South West | +5–15% |
| East of England | +5–10% |
| West Midlands | Baseline |
| East Midlands | Baseline |
| North West | –5–10% |
| Yorkshire and Humber | –5–10% |
| North East | –10–15% |
So a mid-range 20m² extension that costs £48,000 in the West Midlands might cost £55,000–£62,000 in outer London and £65,000–£72,000 in inner London.
What drives the cost up?
Some cost increases are unavoidable. Others are choices. Understanding the difference helps you budget realistically.
Unavoidable cost factors
Ground conditions. If the soil is clay, has trees nearby, or has poor load-bearing capacity, your foundations will be deeper and more expensive. Piled foundations can add £5,000–£15,000.
Access. If your property has restricted access (no side passage, narrow alley), getting materials to the rear of the house costs more. A builder may need to carry materials through the house or use a crane.
Party walls. If you are mid-terrace with neighbours on both sides, you may need two party wall awards. Budget £1,500–£3,000.
Drainage diversions. If there is a public sewer running through your garden (check the water company’s sewer maps), you may need to divert it or build over it with special approval. This can add £3,000–£10,000.
Cost choices
Kitchen. This is the single biggest variable. A basic kitchen starts at £5,000. A mid-range kitchen with integrated appliances runs £10,000–£20,000. A high-end kitchen with stone worktops and premium appliances can exceed £30,000.
Bi-fold or sliding doors. A set of aluminium bi-fold doors typically costs £3,000–£8,000 depending on the width and specification.
Underfloor heating. Wet underfloor heating adds £2,000–£4,000 for a typical extension. Electric underfloor heating is cheaper (£500–£1,500) but costs more to run.
Vaulted or lantern roof. A flat roof with a roof lantern adds £2,000–£5,000. A vaulted ceiling with exposed rafters adds more to the structural cost.
Flooring. Engineered oak: £2,000–£5,000. Large-format porcelain tiles: £2,000–£4,000. Polished concrete: £3,000–£6,000.
Hidden costs most people forget
These are the costs that do not appear in a builder’s quote but still come out of your pocket:
- Skip hire: £250–£400 per skip, and you may need 3–5 during the build
- Scaffolding: £500–£1,500 (sometimes included in the builder’s price)
- Temporary kitchen: If your kitchen is out of action, budget for eating out or a temporary cooking setup
- Garden reinstatement: The builder will leave a building site. Expect £500–£3,000 to restore your garden
- Decoration upgrades: Once the new extension is finished, the existing rooms often look tired by comparison. Many homeowners end up redecorating adjacent rooms
- Building insurance: Inform your insurer about the work. Some policies require notification during construction and an update to the sum insured afterwards
- Council tax reband: A significant extension can trigger a council tax revaluation when you sell (not while you live there)
Budget an additional £3,000–£8,000 for hidden costs on top of the build and professional fees.
A realistic total budget
Putting it all together for a typical mid-range 20m² single-storey rear extension in the Midlands or South:
| Cost element | Amount |
|---|---|
| Build cost (20m² × £2,200/m²) | £44,000 |
| Architect / designer | £2,500 |
| Structural engineer | £500 |
| Building regulations | £600 |
| LDC application | £274 |
| Party wall surveyor (one neighbour) | £1,000 |
| Kitchen (mid-range) | £12,000 |
| Bi-fold doors | £5,000 |
| Hidden costs (skips, garden, etc.) | £4,000 |
| Total | £69,862 |
For London, add 20–40%. For a basic specification without a kitchen, the total drops to around £35,000–£45,000.
How to save money
- Get three quotes minimum. Prices vary enormously between builders.
- Fix the specification before you get quotes. Changing your mind during the build is the fastest way to blow the budget.
- Consider a Building Notice instead of Full Plans for straightforward extensions — it saves on upfront drawing costs.
- Do your own decorating. Painting and finishing can save £2,000–£4,000 in labour.
- Time your build wisely. Builders are busiest in spring and summer. Starting in autumn or winter may get you a better price.
- Use your PD rights if possible. Avoiding a planning application saves £548 and 8 weeks.
Next steps
- Check if your extension is permitted development — building under PD saves time and money
- Read our rear extension complete guide for PD rules, building regulations, and timelines
- Protect your build — our PD Certificate Report helps you apply for an LDC
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