Seek professional advice from an architect or a builder to determine if the work you are hoping to do is possible. Run a planning search for the street. This is an easy thing to do – all of the councils have a planning search facility that works by typing in an address. It will let you see the precedents for work similar to what you plan to do. You’ll also be able to see any conditions on similar developments. If some of the houses on the street have had dormer windows refused, for example, it’s likely this is not something you will be able to do.