Permission is required for “development” which is basically building, engineering, mining or other similar operations or a material change of use of a building or land. There are various pieces of legislation which mean that certain forms of development do not require planning permission.
The main one is the Town and Country Planning (General Permitted Development) Order 2015 which effectively grants a nationwide planning permission for many minor forms of development, e.g. small extensions to some commercial premises, temporary uses of open land and converting the space above a shop to up to two flats. The provisions in the Order most frequently encountered are the rights to alter and extend houses and to build outbuildings and other structures within their gardens. Details of these provisions can be found on the Planning Portal website - Do you need Permission?
There is also the Town and Country Planning (Use Classes) Order 1987 (as amended) which specifies certain classes of use and changes of use from one use to another in the same class does not require planning permission, such as a change of use from type of shop to another. Further details of the provisions of the Use Classes Order can be found on the Planning Aid Direct website.
The legislation regarding what requires planning permission is quite complicated and, if you are unsure whether what you have undertaken required permission, contact Planning Aid England on 0330 123 9244 or speak to a planning consultant.