I used to live in the Cotswolds and many barn conversions were done there including several by friends. One problem is how much the local planning committee want you to stick to keeping the exterior as unaltered as possible. A problem with barn conversions, particularly if it is a large one, is if you are not allowed to include all the windows you would like, they can be a bit dark to gloomy inside. Typically there is one very large window though stretching onto two floors where the original barn doors were.