If you want to change the use of a building, then planning permission may well be needed before the new use is brought into operation. For example, planning permission is needed if you have a large house and want to split it up into flats. If you want to convert an old agricultural building into a house, this is also a change of use and you will need to apply for planning permission. You may want to run a small business from part of your house, perhaps repairing other people's cars. This is also a change of use.