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.
If you want to be certain that the existing use of a building is lawful for planning purposes or that your proposal does not require planning permission you can apply for a "Lawful Development Certificate" (LDC).
The failure to obtain planning permission or comply with the details of a permission is commonly known as a 'planning breach'
You can make certain types of minor changes to your house without needing to apply for planning permission. These are called "permitted development rights"
There are many factors that will affect whether or not you need to apply for planning permission or affect your chances of gaining planning approval.
