Some 18% have added one in the last three years, and 30% will in the next three. This is another huge job, but with more people struggling to move and deciding to improve instead, it's increasingly popular. The amount it costs will depend on an enormous number of things, from the area you have to work with, to the size of the extension. However, assuming you add a single room you could spend around £20,000. HSBC estimates it will add around £15,500 to the value of the property, so you are unlikely to gain as much as you spend.