According to Checkatrade, a single-storey extension can add around 5–8% to the value of your house, while a double-storey build can give a boost of up to 12%.
Before you get the builders in, it's a good idea to evaluate whether the estimated cost of the extension will be lower than the potential added value, which will depend on your area. However, building an extra bedroom or transforming the kitchen into a spacious and practical kitchen-diner is pretty much guaranteed to increase your asking price.
For example, this kitchen extension, featuring a preloved ProNorm kitchen from Rehome, is the perfect light-filled family hub thanks to its glazed gable and bi-folding doors.