Single storey extension
Usually higher per m² than a double storey extension because foundations, roof and site setup are carried by one level only. Use the single storey extension calculator for a specific estimate.
Extension cost benchmarking
Extension Cost per m² Calculator UK
Estimate extension cost per square metre and compare different project types using floor area, specification, complexity and region. This calculator helps you sense-check whether a budget looks realistic before requesting quotes.
This calculator sits inside the extensions cost section. If you already know the project type, use the single storey, double storey, loft conversion, garage conversion or basement conversion calculator for a more specific estimate.
Use this calculator as a benchmark, not a quote
Cost per m² is useful for comparing options, but it should not be treated as a final price. Some costs are fixed, some scale with size, and some only appear once the site is inspected. CostIntel uses this page as the benchmarking layer inside the wider extensions cost cluster.
Compare two project types
This quick comparison helps show the difference between lower total cost and better cost-per-m² efficiency. Use it when deciding between two ways to create usable space.
m²
m²
Comparison result
Option A estimate£57,000–£90,000
Option B estimate£16,200–£32,400
Lower total costOption B
Lower cost per m²Option B
The garage conversion is likely to cost less overall and per m², but the single storey extension may create a larger new living area and change the layout of the home more significantly.
What cost per m² actually means
Cost per m² is the estimated project cost divided by the usable finished floor area. It helps compare different options and can show when a quote looks unusually low or high.
The number is most useful as a warning signal. It does not explain why the cost is high, and it does not replace project-specific pricing. A basement, loft and garage conversion can all have similar floor areas but completely different risks.
Typical UK extension cost per m² ranges
These ranges are planning benchmarks. Use them beside the calculator result and the more specific project calculators in this section.
| Project type | Typical cost per m² | Why the range moves |
|---|---|---|
| Garage conversion | £900–£1,800 | Existing structure can reduce cost, but damp, insulation and services can lift it. |
| Loft conversion | £1,600–£3,000 | Roof structure, stairs, dormers and en-suites affect price. |
| Double storey extension | £1,750–£2,800 | Shared foundations and roof work can improve efficiency. |
| Single storey extension | £1,900–£3,000 | Groundworks, external walls, roof and fit-out carry the cost across one level. |
| Basement conversion | £2,500–£6,000+ | Waterproofing, excavation, drainage and structure can dominate. |
Why small projects often cost more per m²
A small project still needs setup, design, access, waste removal, electrics, insulation, structural work and finishing. These costs are spread across fewer square metres, so the average can look high.
This is why a 15m² side return, a single garage conversion or a small loft can look expensive per m² even when the total cost is lower than a larger extension.
For specific small-project estimates, use the single storey extension cost calculator, garage conversion cost calculator or loft conversion cost calculator.
Why larger projects may look cheaper per m²
Larger projects can reduce the average cost per m² because site setup, design and access are spread across more area. That does not mean the project is cheaper. The total budget can still be much higher.
This is common with double storey extensions, where foundations, roof work and site setup are shared across two levels. If you are comparing two-floor space, use the double storey extension cost calculator.
Cost per m² by project type
The same £/m² number can mean different things depending on the type of project. A low figure is not automatically good, and a high figure is not automatically wrong.
Double storey extension
Often better cost efficiency per m² because key structural costs are shared across two floors. The total cost is still higher. Use the double storey extension calculator to compare two-floor options.
Loft conversion
Cost per m² needs care because sloped ceilings and staircase space reduce practical room area. Use the loft conversion calculator when roof-space suitability matters.
Garage conversion
Can be low per m² when the structure is dry and suitable. The figure rises when insulation, damp work, floor upgrades or services are needed. Use the garage conversion calculator to check the project more closely.
Basement conversion
The least reliable project for simple £/m² benchmarking because waterproofing and structural work can dominate the budget. Use the basement conversion calculator for risk-based pricing.
What to prepare before getting quotes
Before asking builders for quotes, decide which floor area you are using. For extensions, this is usually internal usable floor area. For lofts and basements, remove awkward or restricted space from your mental calculation so the £/m² figure reflects the room you can actually use.
Also separate base build costs from specification choices. Glazing, kitchens, bathrooms, waterproofing, heating and higher finishes can make two projects with the same area produce very different cost-per-m² figures.
Quote-ready benchmark summary
Example brief: 30 m² single storey extension, standard specification, typical complexity, Rest of England pricing, VAT included and 10% contingency.
Related extension cost calculators
Use the cost per m² result to sense-check your budget, then move into the calculator that matches the project type.
Home extension
Broad estimate across common UK extension types.
Single storey extension
Estimate one-level rear, side and kitchen extension costs.
Double storey extension
Estimate two-floor extensions and compare efficiency.
Loft conversion
Estimate roof-space conversion costs and suitability.
Garage conversion
Estimate costs where existing structure can be reused.
Basement conversion
Estimate below-ground conversion cost and risk.
Extension cost per m² FAQs
What is the average extension cost per m² in the UK?
Many UK extension projects sit between around £1,750 and £3,000 per m² before VAT and contingency. Garage conversions can be lower, while basement conversions can be much higher.
Why do small extensions cost more per m²?
Small projects still need setup, access, waste removal, structural work, services and finishes. These costs are spread across fewer square metres.
Is cost per m² a reliable way to compare quotes?
It is useful, but incomplete. Quotes also depend on structure, access, site conditions, specification and whether the measured area is genuinely usable.
Which extension type is cheapest per m²?
Garage conversions are often cheapest where the structure is dry and suitable. Costs rise when insulation, damp work, floor upgrades or services are needed.
Why are basement conversions expensive per m²?
Waterproofing, excavation, drainage, structure and access can dominate the budget. These costs can be high even before the room is fitted out.
Is a double storey extension cheaper per m²?
Often, yes. Foundations, roof work and site setup are shared across two floors. The total project cost is still usually higher than a single storey build.