Rather think there are many Unauthorised works done, more often than not because of not understanding the needs, and probably perfectly good renovations, and wonder is it only when a property is then offered for sale that this requirement is revealed.
Perhaps this could be better advised when a property is originally purchased, as a necessary advice to the new owner, who would then better understand the legislated requirements for structural changes to a property, including major internal works.
Just a suggestion but it would at least give the new owner some knowledge of council requirements.