Cost Of A Double Storey Side Extension
A double-storey side extension typically costs between £60,000 and £120,000, though prices can vary significantly depending on factors such as location, size, design complexity, and the quality of finishes.
In this article, we’ll break down the key factors that influence the cost of a double-storey side extension and offer practical tips to help you manage your budget and reduce unnecessary expenses.
Average Cost Of A Double Storey Side Extension (Guide Only)
| Item | Estimated Cost (£) |
|---|---|
| Design, Planning & Drawings | 3,000 – 8,000 |
| Structural Engineering | 1,500 – 3,500 |
| Foundations & Groundworks | 8,000 – 15,000 |
| Brickwork & Structure | 15,000 – 30,000 |
| Roofing & Windows | 8,000 – 15,000 |
| Plumbing & Electrics | 4,000 – 10,000 |
| Fixtures & Finishes | 10,000 – 25,000 |
| Contingency (10–15%) | 6,000 – 12,000 |
How Size , Layout and Design Impacts the Cost Of A Double Storey Side Extension
Unsurprisingly, the exact size of your double-storey side extension can play a significant role in cost. Simply put: the larger the extension, the more materials and labour you’ll need. What may surprise some however is the additional cost that can come from complex layouts and designs.
The simplicity or complexity of your extension has a significant impact on cost. Straightforward rectangular structures are typically more affordable to build, while designs that include angled walls, integrated glazing, balconies, or bespoke features are more time-consuming and resource-intensive.
Having a well-designed, space-efficient layout can help you to maximise your home’s potential without inflating your build costs.
Planning and Permissions
- Many double-storey side extensions require planning permission. This can be due to various factors such as:
- The side extension being visible from the front of the property. This will impact the street scene or neighbouring boundaries.
- The side extension exceeding local limits due to height, overall footprint, or proximity to the boundary.
- The extension altering the character of the original listed property in a way that falls outside of permitted development rights.
Each local authority has its planning policies, so it’s important to check early in the process. Getting clear, accurate drawings and advice can help ensure your application is successful and avoids unnecessary delays which can contribute to increased costs.
How Location and Regional Pricing Affects Cost Of A Double Storey Side Extension
Build costs vary by region. Unsurprisingly, labour and material prices tend to be higher in London and the South East, while areas in the North and Midlands may be more cost-effective.
It’s also worth factoring in how easily your property can be accessed by tradespeople and suppliers.
Tight side access or restricted parking can increase labour time and costs.
Foundations and Ground Conditions
Groundworks can take up a large portion of the budget – particularly if your soil type, drainage setup, or existing foundations require additional excavation or reinforcement.
If the extension is over drains or near trees, this will increase complexity and impact cost.
This is why it is important to have early site assessments. These assessments can identify potential issues, allowing for more accurate budgeting and fewer surprises during construction.
Internal Fit-Out and Finishes
The final cost of your extension will depend heavily on your chosen finishes. High-spec kitchens, bathrooms, flooring and glazing can significantly increase the overall spend. If you’re adding new bathrooms or utility spaces, you’ll also need to account for plumbing, electrics, and heating integration.
Even within a fixed budget, thoughtful design choices can help prioritise quality in the areas that matter most – without overextending financially.
There are several ways to manage your budget effectively:
- Stick to a simple shape and roof structure
- Limit the number of internal rooms and plumbing-intensive areas
- Use cost-effective finishes where possible
- Avoid frequent design changes during the build
- A clear plan from the start can make all the difference.
Need help planning your side extension?
By thinking through the space, flow, structure and materials early on, you’re more likely to get an extension that works not just on paper, but in real life.
Planning Design London offers design-led support tailored to your home, your goals, and your budget. Get in touch to book an initial consultation and take the first step with confidence.
