Extension type
Single-storey, double-storey, wraparound and side-return extensions each have different structural complexity and base costs.
House Extension Cost Calculator
Enter your extension type, dimensions and a few project details to get an instant UK cost estimate - no signup or personal details required.
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Estimated Budget Range
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How It Works
A three-step process to financial clarity.
1
Enter your details
Select your extension type and provide basic dimensions, specification preferences and your UK region.
2
We calculate
The calculator processes your inputs against UK cost data to generate a tailored estimate for your project.
3
View your estimate
Get a clear cost breakdown you can use for planning, budget discussions and comparing builder quotes.
What the Estimate Includes
Comprehensive breakdown of your architectural costing report.
Core build cost
The estimate covers labour and materials for the main extension structure based on your dimensions and specification level.
Regional adjustment
UK labour rates vary by location so the estimate accounts for area-based cost differences in your region.
Specification level
The estimate adjusts for your chosen finish level from standard through premium to luxury.
Roof type
Your roof choice affects structural and material costs which the estimate factors into the total.
Professional fees
The estimate can include allowances for planning drawings, building control approval and architect input.
Contingency allowance
A contingency margin is included in the final figure to help account for unexpected costs during the project.
What Affects the Cost
Understanding the variables that impact your budget.
Size and layout
The overall footprint affects materials, labour and foundations. A simple rectangular shape is easier to estimate than an irregular or split-level design.
Ground conditions and access
Sloping sites, poor soil or restricted access for machinery can increase groundwork complexity and overall project cost.
Specification choices
High-end finishes, bespoke joinery and premium glazing push costs higher than standard off-the-shelf options.
Site constraints
Party wall agreements, listed building status or limited working space can add time and cost to a project.
What Can Change Your Final Price
Key variables that refine your extension investment estimate.
Size (m²)
Larger floor areas need more materials and labour so the estimate scales with your total square metres.
Specification level
Moving from standard to premium or luxury finish changes the cost per square metre in the calculator.
Region
Labour and material costs vary across UK regions from London and the South East to the North and Scotland.
Roof type
Flat roofs, pitched roofs and glazed roofs each carry different material and structural cost implications.
Professional costs
Including architect fees, planning applications and building control approvals in your estimate is optional.
Frequently Asked Questions
Quick answers to common questions about our estimating process.
How accurate is the estimate?
The estimate is based on typical UK project costs and your inputs. It is designed as a reliable planning guide, not a fixed quote. Actual build costs depend on factors the calculator cannot fully predict, including your specific site conditions, the contractors you choose, material price fluctuations and any changes you make to the design after the initial estimate.
Most homeowners find the figure useful as a starting point for budget discussions and for comparing quotes from builders. For the most accurate picture, we recommend getting at least three detailed quotes from local contractors once you have planning permission and a confirmed specification.
Do I need to sign up or share my details?
No, you can use the calculator straight away without creating an account or sharing any personal information. The tool works purely on the project details you enter and does not ask for your name, email address, phone number or any payment information.
There is no registration step and no requirement to log in before or after you see your estimate. If you want to return later and run the numbers again with different inputs, you can simply open the site and start fresh.
Can I use this before speaking to a builder?
Absolutely. The estimate is designed specifically for the early planning stage, before you approach builders for formal quotes. Having a realistic cost range in mind helps you set a sensible budget, narrow down your options and ask more informed questions when you do speak to contractors.
Just keep in mind that the estimate is a planning tool and you will still need detailed site quotes from individual contractors before committing to a project or agreeing a final budget.
What is the difference between specification levels?
The calculator offers three finish levels to reflect how much you plan to spend on materials and fittings. Standard uses budget-conscious choices such as basic kitchen units, simple sanitaryware and standard double-glazed windows. Premium steps up to mid-range brands, better-quality flooring, integrated appliances and higher-performance glazing. Luxury covers top-end finishes including bespoke joinery, natural stone worktops, underfloor heating and premium bathroom fittings.
Most homeowners choose premium for the main living spaces and standard for utility rooms or less visible areas. The specification level you pick changes the cost per square metre significantly so it is worth trying all three in the calculator to see how your budget stretches.
Does the estimate cover planning permission or architect fees?
The calculator gives you the option to include professional and approval costs in your estimate. This covers typical outlay for an architect or architectural technician to draw up plans, submit a planning application and handle building regulations approval.
If you choose to include these costs, the estimate adds an allowance based on standard UK professional fee ranges. If you leave it out, the figure reflects build costs only and you should budget separately for professional services.
What is the most expensive part of an extension?
For most UK extensions the largest single cost is the shell construction, which includes the foundations, walls, floor slab and roof structure. This typically accounts for around forty to fifty percent of the total build cost because it involves the heaviest materials and the most labour-intensive work.
The next biggest cost driver is usually the glazing, particularly if you choose bi-fold doors, large picture windows or roof lanterns. Kitchens and bathrooms follow closely behind as they combine plumbing, electrics and higher-cost fittings into a small area.