If a half-wall is tall enough (one of the taller half-wall options) you can fit a door from the "short wall" (normal walls) category on it and it functions. It definitely counts as its own room. I use this to make restroom stalls in public buildings. The are counted as separate rooms, so sims don't get embarrassed, but the walls are lower than the main walls, so they look like stalls. The "half-wall" just has to be barely tall enough to fit the door and it works.