Planning Permission, in simple terms, is like asking if you can do a certain piece of building work. It will be granted (possibly subject to certain conditions) or refused.
Parliament has given the main responsibility for planning to local planning authorities (usually, this is the planning department of your local council). Therefore, if you have any queries about a particular case, the first thing to do is to contact your local planning authority.
It is your responsibility for seeking, or not seeking, planning permission. If required, it should be granted before any work begins.