Planning law is notoriously complex and disputes can go on for years. They also vary from one authority to another.
My guess would be that the information on the website is correct, but it's irrelevant unless the local authority decides to take action or a neighbour complains.
I've known a few people who have found themselves in breach of planning laws, who have then been able to arrange retrospective permission. Meanwhile, others have had to pull down unauthorised structures.
There's a man who lives near me, who's been in dispute with the council for decades about an unauthorised shed/annexe. He has a blog and the story is never-ending. Personally, I think he's a bit of a nutter, but his run ins with the council are quite amusing.