Double Storey Extension Cost Calculator
Use our free and detailed calculator to estimate the total project budget for your building project. Understanding the potential double storey extension cost is the first step in effective planning.
| Component | Estimated Cost | Description |
|---|---|---|
| Core Build | £0 | Foundations, walls, roof, basic structure. |
| Kitchen(s) | £0 | Cost for fitting new kitchen spaces. |
| Bathroom(s) | £0 | Cost for fitting new bathrooms/en-suites. |
| Subtotal (Build) | £0 | Total cost before fees and VAT. |
| Professional Fees | £0 | Architect, structural engineer, planning. |
| Contingency Fund | £0 | Recommended buffer for unexpected issues. |
| VAT | £0 | Standard 20% on build costs and fees. |
| Total Project Cost | £0 | The final estimated budget. |
What is a Double Storey Extension Cost?
A double storey extension cost refers to the total financial outlay required to build a two-level addition to an existing property. This isn’t just the price of bricks and mortar; it’s a comprehensive budget that includes materials, labour, professional fees, taxes (VAT), and a contingency fund for unforeseen expenses. Homeowners, property developers, and investors calculate this cost to determine the feasibility of a project, secure financing, and manage the budget effectively from concept to completion. Many people incorrectly assume the cost is simply double that of a single storey extension. However, since the foundations and roof are shared across two floors, a double storey extension cost per square metre is often more economical.
Understanding the full double storey extension cost is crucial for anyone looking to significantly increase their living space without moving. It’s particularly relevant for growing families needing extra bedrooms and bathrooms, or those wanting to add a large kitchen-diner on the ground floor with a new master suite above. A common misconception is that initial builder quotes represent the final price. In reality, these often exclude professional fees (architects, structural engineers), planning application fees, and VAT, which can add 30-40% to the total bill. A thorough calculation is the only way to avoid surprise expenses.
Double Storey Extension Cost Formula and Mathematical Explanation
The calculation for the total double storey extension cost is a multi-step process that builds upon a base cost. The core principle is to establish a build cost and then add the necessary associated costs.
Step 1: Calculate the Base Build Cost. This is the foundational figure, derived from the size, quality, and location.
Base Cost = (Area in m² × Cost per m² × Location Multiplier)
Step 2: Add Costs for High-Value Rooms. Kitchens and bathrooms have high costs for plumbing, electrics, and fittings, which are added separately.
Total Build Cost = Base Cost + (Number of Kitchens × Cost per Kitchen) + (Number of Bathrooms × Cost per Bathroom)
Step 3: Factor in Professional Fees and Contingency. These are calculated as percentages of the Total Build Cost. Professional fees typically range from 10-15%, and a contingency of 10-15% is essential.
Subtotal = Total Build Cost + (Total Build Cost × Fees %) + (Total Build Cost × Contingency %)
Step 4: Apply VAT. The final step is to add Value Added Tax (VAT), which is 20% in the UK, to the subtotal.
Final double storey extension cost = Subtotal × 1.20
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Area | Total internal floor space of both new floors | m² | 30 – 100 |
| Cost per m² | The build cost based on the quality of finish | £/m² | £1,800 – £2,800+ |
| Location Multiplier | A factor adjusting for regional price differences | Multiplier | 0.9 – 1.4 |
| Professional Fees | Cost of architects, engineers, and surveyors | % | 10% – 15% |
| Contingency | A fund for unexpected costs during the build | % | 10% – 15% |
| VAT | Value Added Tax on construction services and materials | % | 20% |
Practical Examples (Real-World Use Cases)
Example 1: Family Home Extension in the Midlands
A family wants to add a 60m² double storey extension to their home in Birmingham. They opt for a ‘Good’ quality finish. The ground floor will be a large open-plan living area, and the first floor will house a new master bedroom and an en-suite bathroom.
- Inputs: Area = 60m², Quality = Good (£2,200/m²), Location = Midlands (1.0x), Bathrooms = 1.
- Calculation:
- Build Cost = (60 * 2200 * 1.0) + (1 * 5000) = £132,000 + £5,000 = £137,000
- Fees (15%) = £20,550
- Contingency (10%) = £13,700
- VAT (20%) = (£137,000 + £20,550 + £13,700) * 0.20 = £34,250
- Final Estimated Cost: £137,000 + £20,550 + £13,700 + £34,250 = £205,500. This figure provides a realistic budget for their planning and financing discussions, demonstrating a comprehensive double storey extension cost.
Example 2: High-Spec Extension in London
A couple in a London suburb plans a premium 80m² double storey extension. They want an ‘Excellent’ quality finish, including a high-end kitchen on the ground floor and two new bathrooms upstairs. Their goal is to maximize both space and property value.
- Inputs: Area = 80m², Quality = Excellent (£2,800/m²), Location = London (1.4x), Kitchens = 1, Bathrooms = 2.
- Calculation:
- Build Cost = (80 * 2800 * 1.4) + (1 * 15000) + (2 * 7000) = £313,600 + £15,000 + £14,000 = £342,600
- Fees (15%) = £51,390
- Contingency (10%) = £34,260
- VAT (20%) = (£342,600 + £51,390 + £34,260) * 0.20 = £85,650
- Final Estimated Cost: £342,600 + £51,390 + £34,260 + £85,650 = £513,900. This higher double storey extension cost reflects the location, size, and luxury specification. Planning for a property value increase is a key part of this investment.
How to Use This Double Storey Extension Cost Calculator
This calculator is designed to provide a robust estimate for your project. Follow these steps to get a reliable budget forecast.
- Enter the Total Floor Area: Input the combined internal area for both new floors in square metres. This is the single biggest factor in the overall double storey extension cost.
- Select the Build Quality: Choose between Basic, Good, and Excellent. This selection adjusts the cost per square metre (£/m²) to reflect the standard of materials, fixtures, and finishes.
- Set the Project Location: Labour and material costs vary across the UK. Selecting the correct region applies a multiplier for a more accurate estimate.
- Specify New Kitchens and Bathrooms: Add the number of new kitchens and bathrooms. These rooms have high ancillary costs and are priced separately from the general per-square-metre rate.
- Review the Results: The calculator instantly displays the estimated total project cost, along with a breakdown of the core build cost, professional fees, contingency, and VAT. This detailed view helps in understanding where the money goes. Check the table and chart for a visual breakdown. The goal is a clear picture of the final double storey extension cost.
Use the ‘Reset’ button to clear inputs and start again. The ‘Copy Results’ button is useful for saving the estimate to share with partners, architects, or financial advisors. A good construction cost calculator like this is an invaluable planning tool.
Key Factors That Affect Double Storey Extension Cost Results
Several critical elements can significantly influence the final double storey extension cost. Understanding these factors is key to managing your budget effectively.
1. Size and Complexity
The larger the extension, the higher the material and labour costs. However, the cost per square metre can decrease on very large projects. Complex designs with unusual shapes, large open spans requiring significant steelwork, or split levels will always cost more than a simple rectangular extension.
2. Quality of Finishes
This is a major variable. Basic finishes (standard laminate flooring, budget-friendly tiles, simple decoration) are far cheaper than high-end options (hardwood floors, imported stone, bespoke joinery, premium appliances). This choice can alter the final double storey extension cost by tens of thousands of pounds.
3. Location
Building costs in London and the South East are significantly higher (up to 40%) than in the North of England or Wales. This is due to higher labour rates and logistical challenges. Every renovation cost estimate must account for this.
4. Ground Conditions and Access
Poor ground conditions (e.g., clay soil, high water table, nearby trees) may require deeper or more complex foundations, adding thousands to the cost. Similarly, difficult site access can increase labour costs and require specialised equipment for material delivery.
5. Extent of Glazing
Large bi-fold doors, skylights, and feature windows are popular but expensive. A wall of glass can cost many times more than a standard brick wall with a simple window. The specification of the glass (e.g., triple-glazed, solar control) also impacts the price.
6. Professional Fees & Permissions
Don’t underestimate the cost of professionals. Architect fees, structural engineer calculations, planning applications, building regulations inspections, and party wall agreements can collectively add 15-20% to the build cost. The total double storey extension cost must include these non-negotiable items.
Frequently Asked Questions (FAQ)
1. Is a double storey extension cheaper than moving?
Often, yes. When you factor in stamp duty, estate agent fees, legal costs, and removal expenses, the total cost of moving can be substantial. A double storey extension can provide the space you need for a lower overall outlay, especially if you love your current location. Compare the total double storey extension cost to moving costs to make an informed decision.
2. How much more is a two-storey extension than a single-storey?
A double storey extension is not twice the price. Because you are using the same foundations and roof for two floors, the second storey adds approximately 50-75% more to the cost of a single-storey extension of the same footprint. This makes the overall two-storey extension cost per square metre much better value.
3. Do I always need planning permission?
Not always, but it’s very likely for a double storey extension. Under Permitted Development rights, some two-storey extensions are possible without a full planning application, but the rules are very strict regarding height, depth, and proximity to boundaries. Always check with your local planning authority.
4. What are the “hidden” costs I should budget for?
Key hidden costs include: re-landscaping the garden, internal re-configuration of the existing house, temporary accommodation if you need to move out, increased insurance premiums, and decorating/furnishing the new space. Your contingency fund is designed to cover some of these.
5. How long does a double storey extension take to build?
For an average-sized extension (50-70m²), the construction phase typically takes 12-16 weeks. However, the design and planning phase before any work starts can take 3-6 months. The total timeline from hiring an architect to completion can be 9-12 months.
6. Can I live in the house during the build?
It’s often possible but can be challenging. There will be significant noise, dust, and disruption. For major works, especially those involving the removal of the back wall of the house, moving out for a portion of the build might be necessary for safety and sanity.
7. How much value will a double storey extension add?
This varies, but a well-designed extension that adds bedrooms and improves living space can add up to 20% to your property’s value. The return on investment depends on the ceiling price for properties in your area and the quality of the extension.
8. Why is a contingency fund so important in the double storey extension cost?
A contingency fund (typically 10-15% of the build cost) is a financial safety net. It covers unexpected issues like uncovering drainage problems, needing deeper foundations, or discovering asbestos. Without it, your project could be derailed by unforeseen expenses.
Related Tools and Internal Resources
Planning a major home renovation involves many financial considerations. Use our other specialized calculators to build a complete picture of your project’s costs and financial implications.
- Loft Conversion Cost Calculator: If you’re considering extending upwards into the roof, use this tool to estimate the costs involved.
- Kitchen Remodel ROI Calculator: Essential if your extension includes a new kitchen. This helps you balance your home extension cost with potential value uplift.
- Bathroom Renovation Cost Estimator: For accurately budgeting the new bathrooms or en-suites in your two-storey extension.
- Property Valuation Tool: Estimate your home’s current and post-extension value to assess the financial viability of your project.
- Mortgage Overpayment Calculator: See how re-mortgaging or increasing your loan to fund the extension will affect your monthly payments and loan term.
- Stamp Duty Calculator: Compare the total double storey extension cost against the cost of moving to a larger house, including the significant expense of stamp duty.