on permanent structures you can go up to 50% of your rear garden(i think), on temperary ie wooden sheds i'm not sure there's a limit.
i split a house to 2 flats for a mate 16 years ago, local council sussed it last year, only because he was doing it to his new house 2 doors away, bugger

. made him undo work on the new one but couldn't touch the old one.
best to check though don't just take my word for it.
i'm from a place with dreaming spires, n half asleep builders!!