How does Green Belt planning policy work?

Green Belt areas have some of the strictest planning controls, and their planning policy is the polar opposite of planning policy in areas that aren’t designated as such.
Normally the planning system is set up to allow development to proceed unless there is a harmful element to it. However, in Green Belts all development is considered inappropriate and therefore harmful.
The only exceptions to this are those stated in government policy (explained in What type of property developments are approved in Green Belts), or if there are very special circumstances in which planning ought to proceed, even if it’s harmful to the Green Belt.