One major thing I forgot to mention.
I, personally wouldn't do anything at all until I had sought approval. What you plan will probably need planning permission - you are supposed to add fire doors and other such safeguards to loft extensions.
With old improvements though, someone, at some time will have sought permission to do the work, and this will be on file for the house.
When you eventually come to sell the house, a search by the buyer's legal team will show that these improvements were never given approval - making it impossible to sell - or it may sell at a grossly lower price than you hope.