Building an extension is a popular way to add space and value to your home. In 2025, the cost of a house extension in the UK is typically estimated per square metre, ranging from £1,500 to £2,500+ per square metre.
For a basic, single-storey rear extension of around 20sqm, you can expect to pay between £30,000 and £50,000. A mid-range finish with features like bifold doors and a fitted kitchen would push the cost towards the higher end of that scale.
A two-storey extension will not cost double a single-storey one, as the foundations and roof costs are already accounted for. A two-storey extension is often more cost-effective, typically costing around 50-60% more than a single-storey version of the same footprint.
The type of extension also affects the price. A simple rear extension is generally the most straightforward. A side-return extension, popular on Victorian terraced houses, can be more complex due to access and structural work, and may cost more per square metre.
Beyond the basic build cost, you must also budget for professional fees, which can add 10-15% to the total. This includes architects' or designers' fees for drawings, structural engineer's calculations, and building control inspection fees. The final cost of the internal fit-out, such as the kitchen, bathroom, flooring, and decoration, will also be a major part of your overall budget.