If you go ahead with a development like an extension or a new building of some sort and you don't have planning permission then the local authority could ask you to make a retrospective planning application.
This is interesting because although they may say yes you need planning permission, it is not commonly known that it is not against any law to build or change the use of something without first having made a planning application which was approved. What is against the law is that should the local authority decide that no they don't like what you're doing, they can issue an enforcement notice which is where the planning enforcement officer will tell you to stop what you're doing and make a formal application. It only becomes unlawful not to comply with the enforcement officer, not the fact of needing a planning application in the first place.
When a local authority looks at a situation they have to decide whether or not it’s in the interest of the council, owner or neighbours to actually issue an enforcement notice. Sometimes you see things being built even if they haven't been given permission, it is not actually unlawful to do that, it's unlawful not to comply with an enforcement notice and it is then up to the local authority planning department whether they will issue an enforcement notice. Enforcement is a very complicated area which I will go into more detail in another post.