Get planning for a full 2nd house.
Don’t worry about compromising the access to the existing house…. you want to move on anyway. Whilst the access situation might not be desirable for you, it always surprises me what other people are happy living with.
There’s a big extended but run down bungalow at the end of our street, large garden. To me you could put a couple of decent sized detached houses there, with good sized gardens/driveways. They’re applying for planning permission for 6 detached houses which must all be on top of each other. I can understand why the owner is doing it, maximising the space and trying to get best value out of the land. Can’t imagine why anyone would want to buy one though. If they get permission I bet they sell quickly!