Structure and site conditions
Foundations, roof work, steel beams, waterproofing and ground conditions can change the budget before finishes are considered.
Extensions cost hub
Extension Cost Calculators UK
Compare UK extension costs across single storey extensions, double storey extensions, loft conversions, garage conversions, basement conversions and cost per m² benchmarks. Use the calculator that matches your project stage, then sense-check the estimate against related options.
Choose the right extension calculator
Each calculator has a different role. Start broad if the project is still unclear, or choose the calculator that matches the space you want to create.
Best if you are still comparing
Home Extension Cost Calculator
Start here if you want a broad estimate across common extension types before narrowing the project.
Best for ground-floor spaceSingle Storey Extension Cost Calculator
Use this for rear extensions, side extensions, kitchen extensions and ground-floor living space.
Best for two levelsDouble Storey Extension Cost Calculator
Use this when the project adds two levels and you want to understand total cost and cost per m² efficiency.
Best for roof spaceLoft Conversion Cost Calculator
Use this for bedrooms, offices and en-suite rooms where the roof space may be suitable.
Best for existing garage spaceGarage Conversion Cost Calculator
Use this when the garage is dry, structurally sound and could become an office, bedroom, utility or living space.
Best for below-ground spaceBasement Conversion Cost Calculator
Use this for existing basements, lower ground floors or excavation-led projects where waterproofing and structure are major cost drivers.
Best for sense-checkingExtension Cost per m² Calculator
Use this to compare project types by floor area and understand whether an estimate looks low, typical or high.
How extension costs differ by project type
The same floor area can produce very different costs depending on whether you are building new space or converting existing structure.
| Project type | Typical cost behaviour | Main cost pressure |
|---|---|---|
| Single storey extension | Lower total than double storey, often higher per m² | Groundworks, structure, roof and glazing |
| Double storey extension | Higher total, often better per m² | Foundations, roofline and upstairs services |
| Loft conversion | Uses existing roof space | Head height, stairs and roof structure |
| Garage conversion | Often lower cost if structure is suitable | Damp, insulation, floor level and services |
| Basement conversion | Highest variability | Waterproofing, excavation, drainage and structure |
| Cost per m² benchmark | Useful for comparison | Can mislead if usable area is unclear |
Start broad, then narrow the estimate
If you are early in planning, start with the Home Extension Cost Calculator. Once the project shape is clearer, move into the specific calculator for single storey, double storey, loft, garage or basement work. If you already have a quote or rough budget, use the Extension Cost per m² Calculator to check whether the number sits in a realistic range.
Typical UK extension cost ranges
These are broad planning ranges. Use the calculator pages for project-specific inputs such as layout, region, specification, VAT and contingency.
| Project | Typical range | Best calculator |
|---|---|---|
| Garage conversion | £12,000–£75,000+ | Garage Conversion |
| Loft conversion | £25,000–£140,000+ | Loft Conversion |
| Single storey extension | £30,000–£160,000+ | Single Storey |
| Double storey extension | £70,000–£280,000+ | Double Storey |
| Basement conversion | £30,000–£500,000+ | Basement Conversion |
What usually drives extension costs?
Floor area sets the base, but the final price is usually shaped by structure, access, specification, services, region and the condition of any existing space being converted.
Services and specification
Kitchens, bathrooms, heating, glazing, insulation, electrics and flooring can move two similar projects into different cost bands.
Existing-space suitability
Lofts, garages and basements depend on whether the structure can become comfortable usable space without major remedial work.
Building new space vs converting existing space
Single and double storey extensions create new space. They usually involve groundworks, external walls, roof construction, drainage and structural openings.
Loft, garage and basement conversions reuse existing space. The question changes from “how much new space can we build?” to “how suitable is the existing structure?” That is why a garage can be lower cost when dry and sound, while a basement can become expensive if waterproofing and structure dominate.
Cost per m² is useful, but incomplete
Cost per m² is useful for comparing options, but it does not explain the full project. A garage conversion may look cheap per m² if the structure is good. A basement may look expensive per m² because waterproofing and structure dominate the budget. A double storey extension may look efficient per m² even though the total cost is higher.
Use the Extension Cost per m² Calculator when you already have a quote, budget or floor area estimate and want to sense-check it.
Why CostIntel uses ranges, not fixed prices
CostIntel uses ranges because early-stage extension estimates should reflect uncertainty. Ground conditions, access, structure, specification and regional labour rates can change the final quote. A fixed number would look precise but give a weaker planning result.
For more detail, read our methodology, pricing data and how costs are calculated.
Suggested decision paths
Use these paths when the project idea is still forming. They are designed to move from broad estimate to specific calculator without forcing every user through the same route.
I only know I need more space
Start with the Home Extension Cost Calculator, then use the Extension Cost per m² Calculator once you have a budget range.
I want a bigger kitchen or living area
Use the Single Storey Extension Cost Calculator. If an existing garage could solve part of the problem, compare it with the Garage Conversion Cost Calculator.
I need bedrooms or office space
Use the Loft Conversion Cost Calculator if roof space is available. Compare with the Double Storey Extension Cost Calculator if you also need downstairs space.
I want to use existing structure
Compare the Garage Conversion, Loft Conversion and Basement Conversion calculators before deciding whether to build new space.
Extension cost FAQs
Which extension calculator should I use first?
Use the Home Extension Cost Calculator if you are still comparing project types. Use a specific calculator once you know whether the project is single storey, double storey, loft, garage or basement.
What is the cheapest way to add space?
A garage conversion is often one of the lowest-cost options where the structure is dry and suitable. A loft conversion can also be cost-effective where the roof space works well.
Is a double storey extension better value?
It can be better value per m² because foundations and roof work are shared across two levels. The total cost is usually higher than a single storey extension.
Why are basement conversions so expensive?
Basement projects involve waterproofing, drainage, structural work and sometimes excavation. These cost drivers can dominate the budget.
Should I compare cost per m²?
Yes, but use it as a benchmark. Cost per m² is useful for comparing options, but it does not show structure, access, specification or site risk.
Are CostIntel estimates fixed quotes?
No. They are planning estimates based on typical project scopes and cost drivers. Builders need to inspect the property before giving a fixed quote.