Under planning law, types of use are grouped into 'use classes'. Planning permission will usually be needed for change of use from one class to another and when the use of a building or land changes from one use class to another. There are some exceptions where changes are allowed without permission.
For more information, visit the Planning Portal website.
Planning permission is needed to sub-divide a house into flats. Converting a previously sub-divided house back to a single dwelling does not require planning permission unless the alterations affect the external appearance of the property and unless the property is a listed building where the alterations affect the character of the building.