A double-storey extension is ideal for those looking forward to doing something big on their property. The grander the extension, the huge will be the returns. Obviously, this type of extension surely costs some bucks but when you look at the returns, then associated costs start making sense. Generally, a double-storey extension costs up to £76,000. Though this cost is quite high for a one-time investment, but this rightfully pays off when the owner lists the property for either rent or sale. Usually, it is noted that double-storey extensions increase property value by 100% approximately.