The same policy applies if the proposed building is for your granny, your servant, your annoying spotty teen, or your grotesque cousin who must be kept away from villagers and full moons.They should. In reality they have a lot of leeway and I've seen them bend the rules for many a sob story.
If you want an outbuilding for your 20-odd year old offspring you'll probably be refused. If it's for granny then you have a very good chance (then the 20-odd year old moves in later).
The building must be ancillary to the main house, occupied by a relative, helper or employee, and not rented out. That's it.
