Best fit
Rear living spaces, kitchen extensions, garden rooms, side returns and wraparound layouts where ground-floor space is the priority.
Single storey extension costs
Single Storey Extension Cost Calculator UK
Estimate the cost of a ground-floor extension using size, layout, finish level and region. This calculator is designed for rear extensions, side extensions, side returns and wraparound layouts.
This page sits inside the extensions cost calculators section. If you are still comparing different extension types, start with the home extension cost calculator. For assumptions and pricing logic, see our methodology.
Use this estimate before speaking with builders
A single storey extension quote is shaped by more than floor area. Builders need to inspect access, ground conditions, drainage, structural openings and finish level before confirming a fixed price. CostIntel gives you a realistic planning range first, so you can decide whether the idea fits your budget before arranging quotes.
Is a single storey extension the right project type?
A single storey extension is usually the right starting point when you want more ground-floor living space, a larger kitchen, a dining area facing the garden or a better connection between the house and outdoor space.
It may be less efficient when the property has a small plot or when the main need is upstairs space. In those cases, compare the numbers with the double storey extension cost calculator or a loft conversion cost calculator.
Watch the footprint
The project uses garden or side space. That trade-off matters if the property already has limited outdoor area.
Compare if unsure
If you also need bedrooms or bathrooms upstairs, a two-storey option may give better cost efficiency per square metre.
What affects single storey extension cost?
The size of the new space sets the base, but the final quote is usually decided by layout, structure, access and finish. Two 30m² extensions can price very differently if one is a simple rear build and the other is a wraparound extension with large openings, roof lanterns and drainage changes.
Layout and structure
A rear extension is often simpler than a side return or wraparound layout. More corners, larger openings and more steelwork usually increase labour and material costs.
Groundworks and drainage
Excavation, foundations and drainage checks can be a larger share of the budget than expected. Existing manholes, pipes or poor ground can push the project upward.
Finish and fittings
Flooring, glazing, lighting, heating, kitchens and decorating can move the same shell into a much higher final range.
Typical UK single storey extension cost ranges
Use these ranges as a planning reference beside the calculator. They are not fixed quotes. The purpose is to show how layout and size change the budget before you request site-specific pricing.
| Project type | Typical size | Estimated range | Why the range moves |
|---|---|---|---|
| Small rear extension | 15–20 m² | £30,000–£55,000 | Small projects often have higher cost per m² because setup and foundations are spread across less space. |
| Standard rear extension | 25–35 m² | £45,000–£90,000 | The range depends heavily on structure, roof choice, glazing and internal finish. |
| Side return extension | 12–25 m² | £35,000–£80,000 | Narrow access, drainage and structural openings can increase the cost despite modest floor area. |
| Wraparound extension | 35–55 m² | £80,000–£160,000 | Larger footprint, more structure, more roof area and more glazing often lift the budget. |
Where the money goes in a single storey extension
Groundworks and foundations usually arrive early in the budget. Even a modest extension needs excavation, spoil removal, foundation work and checks around drains or services. These costs do not scale down neatly on a small build.
Structure is the next major pressure point. Removing an external wall to create an open-plan kitchen or living area often needs steel beams and structural calculations. Large glazed openings can add cost through both glazing and the structure needed to support it.
The final spread comes from the fit-out. A basic room with plastered walls and standard flooring is very different from a kitchen extension with cabinetry, worktops, lighting, heating, flooring and appliances.
Real-world pricing scenarios
These examples show why the calculator uses a range. The same floor area can sit in different bands once location, access, structure and specification are considered.
25m² rear kitchen extension
A typical Midlands or northern England project with a standard finish may sit around the middle of the estimate. The range moves upward if the rear wall is removed for a wide open-plan layout.
20m² side return extension
The floor area may be modest, but access and drainage can make the cost per m² higher. This is common on terraced properties where working space is tight.
40m² wraparound extension
A larger layout in London or the South East can move into the upper range when it includes large glazing, roof lanterns, kitchen work and higher finish levels.
Why your quote might be higher than the estimate
The largest changes often come from site conditions and specification detail. A quote can rise when builders need more labour time, extra structural design, deeper foundations or specialist trades.
Limited access
Narrow side access, terraced layouts and difficult material handling can slow the build and increase labour time.
Drainage relocation
Moving drains, adjusting inspection chambers or working around services can add groundworks cost before the visible build begins.
Large openings
Open-plan designs, bifold doors and wide glazed areas often need structural steelwork and more detailed engineering.
Higher specification
Kitchens, premium flooring, roof lanterns, underfloor heating and bespoke joinery can move the project well above a basic shell estimate.
Single storey vs double storey extension cost
A single storey extension usually has the lower total cost because it adds one level only. It is often the right choice when the main goal is a larger kitchen, family room or garden-facing living space.
A double storey extension costs more overall, but it can be more efficient per square metre. The project shares foundations, roof work and site setup across two levels. If you need both downstairs and upstairs space, compare this page with the double storey extension cost calculator.
Cost per square metre needs context
Cost per m² is useful for early budgeting, but it can mislead when used alone. A 15m² side return can look expensive per square metre because foundations, design, setup and drainage are spread across a small area.
A larger rear extension may have a lower cost per m² but a much higher total price. For a focused floor-area comparison, use the extension cost per m² calculator.
Why averages move
Small extensionHigher per m²
Standard rear buildMid range
Wraparound layoutHigher total
What to prepare before getting quotes
You do not need a final design before speaking with builders, but a clearer brief helps make quote comparisons more useful. Start with the approximate size, the layout type, whether it includes a kitchen, the finish level you expect and any access issues you already know about.
When possible, separate the building shell from high-cost extras such as kitchens, roof lanterns, bifold doors and landscaping. This makes it easier to see whether the core build is affordable before optional upgrades are added.
Quote-ready summary
Example: 30 m² rear extension, standard finish, typical complexity, Rest of England pricing, VAT included and 10% contingency.
When an alternative project may be worth comparing
Extending outward is not the only way to create usable space. If your budget is tight or the plot is limited, compare the estimate with projects that use existing structure.
Loft conversion cost calculator
Useful when the main goal is an extra bedroom, office or bathroom rather than larger ground-floor living space.
Garage conversion cost calculator
Compare this if the garage has suitable structure, access and services nearby.
Basement conversion cost calculator
Relevant for properties where below-ground space is possible, but waterproofing and structure need careful allowance.
Related extension cost calculators
Use these calculators to move from a broad estimate into a more specific project comparison.
Home extension
Broad estimate across common extension types.
Double storey extension
Compare costs across two levels.
Extension cost per m²
Compare extension costs by floor area.
Loft conversion
Compare converting roof space with extending outward.
Bifold doors
Estimate glazing costs if the design includes large openings.
Kitchen renovation
Useful when your extension includes a new kitchen fit-out.
Single storey extension cost FAQs
How much does a single storey extension cost in the UK?
A smaller simple build may start around £30,000. A typical 25–35m² extension often sits between £45,000 and £90,000, while larger or more complex projects can exceed £120,000.
What is the average cost per m²?
Many single storey extensions sit around £1,900–£3,000 per m² before VAT and contingency. Side returns and wraparound layouts can sit higher because they often involve more structure and access constraints.
Is a kitchen extension more expensive?
Usually, yes. A kitchen extension can add plumbing, electrics, ventilation, cabinetry, worktops, appliances, flooring and lighting to the main build cost.
Why do small extensions cost more per m²?
Some costs do not shrink much with size. Site setup, design, access, foundations and drainage can make smaller builds look expensive on a per-square-metre basis.
Does this calculator include VAT?
The VAT option is switched on by default. You can turn it off when comparing pre-VAT build costs.
Should I compare a double storey extension?
Yes, if you also need upstairs space. A double storey extension usually costs more overall, but can be more efficient per m² because foundations and roof work are shared across two levels.