Ask at the local district council planning dept. to speak to the Conservation Officer.
It's largely up to him. Their opinions vary so much - even though there are supposed to be common guidelines.
I'm converting a barn at the moment, and the Conservation guy doesn't even like Velux rooflights - whereas they are routinely fitted to quite sensitive buildings such as church conversions.
You might get loads of opinion, but, he's the only one who can tell you for sure.
Good luck