Any developments other than those which fall under permitted development, require planning permission, and a retrospective application can be made if work has already commenced on the site. The local authority will process this application in the same way as a normal planning application, and there is still the risk that permission may be refused, which could be damaging to the work already started.
If permission is refused for a retrospective application, this could mean that either modifications have to be made to the work, or in some cases the development has to be returned to the state it was in before the work started. Local authorities can also request a retrospective application if they believe that work has been carried out that should have originally required planning permission. This request does not mean that planning permission will automatically be granted either, so undertaking work without planning permission has a serious risk.