Size and layout
Larger extensions usually cost more overall, but small extensions can cost more per square metre because site setup and design work are spread across less floor area.
Extensions cost calculator
Home Extension Cost Calculator UK
Estimate the cost of a UK home extension using floor area, build type, finish level and region. The result gives a realistic range, not a false fixed price, so you can budget before speaking with builders.
This is the broad extension estimate inside our extension cost calculators section. You can view all extension cost calculators to compare different project types before narrowing your estimate. For assumptions and data handling, see our methodology.
Use the estimate before speaking with builders
CostIntel gives you a budget range before you commit to quotes. A builder will still need to inspect access, ground conditions, structural requirements and the level of finish before confirming a price.
What affects home extension cost?
The size of the extension sets the base, but the final price is shaped by structure, access and finish. A simple rear extension with straightforward foundations will price differently from an open-plan design that needs steel beams, drainage changes and large glazed openings.
Structure and access
Steelwork, difficult access, drainage relocation and foundation depth can move a project above the typical range.
Finish level
Flooring, glazing, kitchens, bathrooms, lighting and internal detailing can create a large spread between two extensions of the same size.
Typical UK extension cost ranges
These ranges are a planning guide. They are most useful when compared against the calculator result for your own size and build type.
| Extension type | Typical size | Estimated range |
|---|---|---|
| Small rear extension | 15–20 m² | £28,000–£50,000 |
| Standard single storey extension | 25–35 m² | £45,000–£85,000 |
| Double storey extension | 40–60 m² | £75,000–£140,000 |
| Side return extension | 12–20 m² | £35,000–£70,000 |
| Wraparound extension | 35–55 m² | £80,000–£160,000 |
Real-world pricing scenario
A 30m² rear extension in the Midlands with a standard finish often sits in a different budget range from the same floor area in London. The Midlands project may fall around the middle of the calculator range, while a London project with large glazed openings, heavier steelwork and higher specification finishes can move sharply upward.
This is why CostIntel shows a range rather than a single number. The calculator is designed to reflect the way extension quotes move when site conditions, region and specification change.
Cost per square metre needs context
Cost per m² is useful for early budgeting, but it can hide fixed costs. A small extension may look expensive per m² because foundations, access, design work and site setup are carried by a smaller floor area.
For a focused floor-area comparison, use the extension cost per m² calculator. This is the best next step when you want to compare extension sizes before choosing a final layout.
Why averages move
Small extensionHigher per m²
Larger extensionLower per m²
Complex structureHigher range
Choose the calculator that matches your extension type
The home extension calculator is best when you are still comparing broad options. If the layout is already defined, a specific extension calculator can give a cleaner range.
Single storey extension cost calculator
Use this when you are planning a rear, side or kitchen extension on one level. It is the closest match for most ground-floor extension projects.
Double storey extension cost calculator
Use this when the extension adds space across two floors. It helps account for the way foundations, roof work and structure are shared across more floor area.
When an alternative project may be worth comparing
Extending outward is not always the only way to create more space. If you are deciding between building new floor area or using existing space, compare this estimate with a loft conversion cost calculator or garage conversion cost calculator before committing to a layout.
Loft conversion cost calculator
Compare this when extra bedrooms or office space could be created within the existing roof structure.
Garage conversion cost calculator
Useful where the garage already has suitable access, structure and services nearby.
Basement conversion cost calculator
Best for properties where below-ground space is possible but structural and waterproofing costs need careful allowance.
What an extension quote usually includes
A typical quote may include groundworks, foundations, walls, roof structure, windows, doors, first fix services, plastering, basic finishes and waste removal. Kitchens, bathrooms, premium glazing, landscaping and planning fees may appear as separate items or allowances.
If the project includes large glazed openings, the bifold doors cost calculator can help separate glazing costs from the main build estimate.
When the final price can rise
The biggest cost changes usually come from site conditions rather than the headline size. Restricted access can slow labour. Drainage changes can add groundwork. Steel beams can affect both material and labour cost. A structural engineer may be needed where walls are removed or large openings are created.
For related professional costs, use the structural engineer cost calculator or read how we handle assumptions in our pricing data.
CostIntel estimates are based on typical UK labour rates, material costs and project structures observed across residential builds.
Related extension cost calculators
These links connect the extension cost cluster. Use the broad calculator first, then move into the page that matches the project shape.
Single storey extension
Estimate one-level rear, side and kitchen extension costs.
Double storey extension
Estimate extension costs across two floors.
Extension cost per m²
Compare extension costs by floor area.
Loft conversion
Compare converting roof space with extending outward.
Garage conversion
Estimate conversion costs where existing structure can be reused.
Basement conversion
Estimate below-ground conversion costs and structural allowances.
Home extension cost FAQs
How much does a home extension cost in the UK?
A small extension may start from around £28,000, while larger or higher specification extensions can exceed £100,000. Size, build type, access and finish level matter more than the headline category.
Is a double storey extension cheaper per m²?
Often, yes. The total cost is higher, but the cost per m² can be lower because foundations, roof work and site setup are shared across more space. Use the double storey extension cost calculator if your project adds two levels.
Does this calculator include VAT?
The VAT option is switched on by default. You can turn it off when comparing pre-VAT build costs.
Why do small extensions cost more per m²?
Some costs do not shrink much with size. Design work, setup, access, drainage and foundations can make smaller extensions look expensive per square metre.