People generally need permission before carrying out any form of 'development'. Development means constructing new buildings or significantly changing how land is being used (for example, changing from agricultural use to retail use or buildings changing from an office to a flat)
Some changes to land and buildings are not classified as 'development' within planning law and therefore do not need planning permission.
This includes agricultural and forestry practices and internal changes to many buildings.
Some proposed development or changes of use count as 'development' but are considered minor and are therefore automatically granted permission (to avoid bureaucracy and clogging up the system).
Some developments are automatically permitted in principle but may need prior approval by the local planning authority for their details before they can be carried out. One example is telecommunications masts less than 15 metres high.
Permitted development is classified through the Town & Country Planning (Use Classes) Order 1987 or through a local development order.
Details vary, but typically there are stricter controls over buildings, tree felling and other works and demolition and more restrictive limitations on the erection of satellite dishes.
Fish farming and extensions to farm buildings are more tightly controlled in national parks.
Not sure if your intended development or a neighbour's intended development needs permission?
Speak to your local planning authority. If development needing permission has begun without receiving it, the authority may put a stop to it. At the very least, it will ask for an application to be submitted for retrospective planning permission.