I (American living in London for more than a decade) don't think I've ever seen a detached single story house before. There might be a name but they're rare enough that I've never heard it before.
Then we would think it's a three-storey building. Really don't see the issue with calling the ground level what it is. The ground floor is zero levels above the ground. The first floor is one level above the ground. Think of it like this: how many flights of stairs does it take to get to that floor?
Example: my local hospital lists a ward I visited as being on the second floor, therefore you go up two flights of stairs to get to it.