In supplement to what bigall said, you also need planning permission if the council has taken away the rights, known as Permitted Development, granted to houses by central government. To ease strain on the planning system, most houses have automatic permission for certain minor works. Councils can remove these rights, usually by an article 4 direction, which affects many houses in an area or by specific conditions on a particular ocular planning permission, which affects a smaller number of houses.
Before building your lean to you should seek to establish whether you do have PD rights. If you do, you can build within the rights you have. If you do not, you need to make a planning application (but there is no fee). If you want to build something that would not be allowed under PD because, say, it is too large, then you need to make an application and pay a fee