Planning permission is required for the “material change of use” of a property. This is a change of the main use of a property, not ancillary uses, e.g. if a shop choses to change part of its customer area to an area where goods to be sold will be stored that is a change of ancillary uses. The Use Classes Order (see the entry below for this) specifies various categories of use and planning permission is not required to change the use of a property from one use to another in the same use class.
