Well this seems to have gone a way I wasn't expecting.

For clarity - I WILL be asking for whatever permissions are necessary as mentioned in my OP when mentioning a structural engineer. Was just after ideas. Since having an issue with a conservatory we had built under permitted development many years ago (all was fine, but our buyers solicitor really tried it on) we are doing everything by the book and triple checking things

In an ideal world I'd love a full conversion but feel the cost, whilst it would increase the value, is too much to fork out for something that isn't fully needed. Additionally, I live behind a farm that has sold some land and we don't know what will come of it yet - so would want to know that before converting to say a master bedroom. There is scope to create office space up there though and have that done properly too - have even thought of splitting into two rooms, office and storage, but again just after ideas. Thanks re: the pinterest pointer. And thanks mk but I live in Sheffield so out of the way.

All points noted however.