EDIT: As per above, I was off on the opening area.
I found the following, though it is a US source.
I think so long as the closet is deep enough to hang clothing and has a door (which implies it is at least 24" wide) then it is a bedroom.
You could also probably buy one of those Ikea wardrobe units and "permanently attach" it to the wall and probably get away with it.
http://www.city-data.com/forum/ralei...oom-house.html