A retrospective planning application is an application for planning permission after work on your development has already commenced. An LPA can request that the owner or occupier of the land makes a retrospective planning application if they have started work without previous approval. This does not guarantee automatic planning permission.
A retrospective planning application will be processed in the same way as a standard planning application. Should planning permission be refused at this stage, the LPA could issue an enforcement notice that requires you to return the site to how it was before works commenced.
NEVER ignore a planning enforcement notice.