My boys are 6 & 7. They had the option of moving into separate bedrooms but they didn't want to so the bedroom that was meant to belong to DS2 is now a spare room. There is nothing wrong with sharing with a sibling. People all over the world do it without any kind of emotional damage. In fact I think that some people would argue that sleeping in a room all alone is less natural than sharing a room.
If you want to extend just make sure that you're buying a house that it won't be too troublesome to extend.