Double Storey Extension Cost Calculator
Estimate the cost of your two-storey extension in the UK with our detailed calculator.
Enter the combined internal floor area for the ground floor and first floor.
Basic (essential finishes), Standard (good quality), Premium (high-end materials and bespoke features).
Costs are significantly higher in London and the South East.
Select if the new space will include a kitchen or bathroom fit-out.
Estimated Project Cost
Including 20% VAT
£0
Base Build Cost
£0
Fit-Out Cost
£0
VAT (20%)
Cost Breakdown Visualization
What is a Double Storey Extension Cost Calculator?
A double storey extension cost calculator is a specialized online tool designed to provide homeowners, developers, and builders with a reliable estimate for building a two-floor extension on a property. Unlike generic construction calculators, this tool is specifically programmed with variables that directly influence the cost of a two-storey project in the UK. This includes factors like regional price differences (e.g., London vs. other areas), the quality of materials, and the inclusion of high-cost rooms like kitchens and bathrooms. Anyone considering adding significant space and value to their home should use this calculator as a first step to understand the potential financial commitment before engaging with architects or builders.
Double Storey Extension Formula and Explanation
Calculating the cost of a double storey extension involves several key variables. Our calculator uses a robust formula to provide a realistic estimate. While a double storey extension is more expensive overall than a single storey one, the cost per square metre is often less because foundational and roofing costs are spread over two floors.
The core formula is:
Total Cost = (Base Construction Cost + Additional Fit-Out Costs) + VAT
Where:
- Base Construction Cost = Total Area (m²) × Base Cost per m² × Quality Multiplier × Location Multiplier
- Additional Fit-Out Costs = Cost of Kitchen + Cost of Bathroom (if selected)
- VAT = 20% of the combined total.
| Variable | Meaning | Unit / Type | Typical Range / Value |
|---|---|---|---|
| Total Area | The total internal floor space across both new floors. | m² (or ft²) | 20 – 100 m² |
| Base Cost per m² | A baseline construction cost for a two-storey build. | Currency (£) | £1,800 – £2,500 |
| Quality Multiplier | Adjusts the cost based on the level of finish. | Multiplier | 1.0 (Basic) – 1.5 (Premium) |
| Location Multiplier | Accounts for higher labour and material costs in expensive areas. | Multiplier | 1.0 (Rest of UK) – 1.3 (London) |
| Kitchen / Bathroom Cost | A fixed estimate for fitting out these complex rooms. For more details, see our kitchen remodel cost calculator. | Currency (£) | £5,000 – £15,000+ |
Practical Examples
Example 1: Standard Extension in Manchester
A homeowner wants to build a 50m² (total area) two-storey extension with a standard quality finish and an additional bathroom.
- Inputs: 50m², Standard Quality, Rest of UK, Add Bathroom.
- Calculation: ((50m² * £1,900/m² * 1.2) + £6,000 bathroom) * 1.20 VAT = £144,000
- Result: The estimated cost would be approximately £144,000.
Example 2: Premium Extension in London
A family in London is planning a large 80m² two-storey extension with a premium finish, including a new high-end kitchen.
- Inputs: 80m², Premium Quality, London / South East, Add Kitchen.
- Calculation: ((80m² * £1,900/m² * 1.5 * 1.3) + £12,000 kitchen) * 1.20 VAT = £367,920
- Result: The estimated cost for this high-spec project would be around £367,920.
How to Use This Double Storey Extension Cost Calculator
- Enter Total Floor Area: Input the total planned internal area for *both floors combined*. You can switch between square metres (m²) and square feet (ft²) and the calculator will convert it automatically.
- Select Build Quality: Choose between Basic, Standard, and Premium. Standard is the most common choice for a good balance of cost and quality.
- Choose Location: Select whether your project is in London/South East or elsewhere in the UK, as this has a major impact on labour costs.
- Add Complex Rooms: Check the boxes if your extension will house a new kitchen or bathroom, as these have significant additional fit-out costs. Explore our bathroom remodel cost guide for more info.
- Review Results: The calculator instantly displays the total estimated cost, including a breakdown of the base build cost, fit-out costs, and VAT. The pie chart also visualizes this breakdown.
Key Factors That Affect Double Storey Extension Cost
The final price of a double storey extension can vary widely based on these critical factors:
- Size of the Extension: This is the most significant cost driver. A larger footprint means more materials, more labour, and more time.
- Specification of Finishes: The choice of windows, doors, flooring, cladding, and internal fixtures can dramatically alter the cost. A premium finish with bifold doors and underfloor heating will be far more expensive than a basic finish.
- Structural Complexity: If the design requires large amounts of structural steel (e.g., to create an open-plan space) or complex roof geometry, costs will increase.
- Site Conditions: Difficult access for machinery, poor ground conditions requiring deeper foundations, or the need to move existing drainage will all add to the budget.
- Location: Labour rates vary hugely across the UK. Expect to pay a premium of 20-30% or more for projects in London and the surrounding area.
- Professional Fees: Don’t forget to budget for architect’s drawings, structural engineer’s calculations, planning application fees, and building control inspections. These can add 10-15% to the total project cost. For a detailed breakdown, consult an architect fee calculator.
Frequently Asked Questions (FAQ)
Yes, generally it is. While the total cost is higher, you are using the same foundation and roof for twice the floor area, making it more cost-effective in terms of cost per m².
No, this calculator provides an estimate for the construction and fit-out costs, including VAT. You should budget an additional 10-15% on top of this figure for architect, structural engineer, and planning fees.
It provides a strong budgetary estimate based on current industry data. However, it’s not a formal quote. The final cost will depend on your detailed design, choice of contractor, and specific site conditions.
This is due to higher labour wages, greater demand, more complex logistical challenges (e.g., parking, access), and higher material delivery costs in the capital.
Basic: Functional but simple finishes, standard uPVC windows, basic electrics and plumbing. Standard: Good quality materials, aluminium windows or better uPVC, more electrical points, better quality kitchen/bathroom fittings. Premium: High-end materials like hardwood flooring, bespoke joinery, large glass elements (bifolds), smart home integration, and designer fittings.
Often, yes. While some smaller two-storey extensions might fall under Permitted Development rights, they are subject to strict rules on height, proximity to boundaries, and overlooking. It is highly recommended to assume you will need a full planning application. You may want to review our guide to planning permission.
A well-designed two-storey extension that adds bedrooms and improves living space can add up to 20% to the value of your property, often exceeding the cost of the project itself, especially in high-value areas.
Once construction starts, a typical project can take 4-6 months, depending on size, complexity, and any unforeseen delays.
Related Tools and Internal Resources
Planning your project involves more than just the main build. Use our other specialized calculators and guides to get a complete picture of your budget.
- Loft Conversion Cost Calculator: Considering going up instead of out? Estimate the cost here.
- New Build Cost Calculator: Planning a larger project? This tool helps budget for building from scratch.
- House Renovation Cost Calculator: Budget for updating the rest of your property at the same time.