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. The meaning of development is defined in Section 55 of the Town and Country Planning Act 1990.
It is important to note the various pieces of legislation which have been enacted to 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.
As a householder you should also be able to contact the duty planning officer at your Local Planning Authority (LPA) to discuss the necessity of obtaining planning permission for proposals to your property. They may advise you to submit an application for a lawful development certificate for an existing or proposed use or development. This is a confirmation certificate that the works or use you are proposing is lawful for planning purposes and does not require planning permission.