you will propably need planninf permission for a new dwelling in the garden, and also for the barn conversion, but more likely to get it for the barn generally, but what ever you do, do not demolish the barn once you get planning permission as often in their policies once that has happened it is as if it was never there.
Have a look on the planning portal website for that area to see what has been done in the surrounding area, and talk to some active architect and plan drawers in the area for advice, and if you get a nice planner they may actually come out of the office to see you.