Renovation costs for a same level extensions are usually between $25,*-$75,; depending on your materials, labour and project management. As a rough. How much will a two storey extension cost? Your bespoke Home and Maker quote shows you - and also how much value it can add to your home. If you want a two-storey extension, you'll have to add roughly 50% to the cost of a single-story extension. For example, if a single-story extension costs.