Adding an extension is one of the best ways to create more space and add value to your home. But before you dive in, understanding the costs involved can save you from budget overruns and unexpected surprises.
This free tool provides an instant estimate of how much your extension is likely to cost—just answer a few simple questions about your location, the type of extension you’re planning, and how much of the work you’d like to handle yourself.
But what really determines the final price? Let’s break it down.
What Affects the Cost of an Extension?
Extension costs depend on a range of factors, from materials to site conditions. Here’s a rough guideline of what you can expect:
- £1,800–£3,400 per m² – Basic construction costs, with high-spec materials pushing the upper end
- £1,600–£2,300 per metre – If you’re adding bi-fold doors
- Smaller extensions (under 15–20m²) cost more per m² due to fixed expenses like foundations and structural work
- Difficult ground conditions – Sloping sites, clay-heavy soil, or trees nearby can increase foundation costs
- Budget split: Typically, 50% goes to construction, while the other half covers interior finishes
“Many homeowners focus on the shell of the extension but forget how much interiors can add up,” says Udi Regev, co-founder of BetterSpace. “High-quality finishes, bespoke features, and last-minute design changes can push costs higher than expected. Planning ahead for these expenses is key.”
How to Get an Accurate Price
To make sure you’re getting the best value, it’s important to compare multiple builder quotes rather than relying on just one or two. Even when the materials and project specs are the same, contractor pricing can vary significantly.
“Transparency is everything when it comes to renovation costs,” says Regev. “At BetterSpace, we help homeowners understand exactly where their money is going. A detailed breakdown of costs ensures you’re not overpaying and that your budget is being spent wisely.”
Plan Smart, Save More
Want to take the guesswork out of budgeting? Use our free cost estimator to get a realistic price for your extension—before you even meet with a builder. With the right planning and expert insights, you can make your dream space a reality without breaking the bank.
