Is a loft a room for purposes of mezuzah?
Consider a cabin that has one large room with a loft extending over part of that room. A half-height wall at the front of the loft separates it from the rest of the cabin, so there is a single common ceiling over all. (Someone standing in the loft can see someone standing in the main room.) Does this loft count as a room for the purposes of requiring a mezuzah, assuming there is a relevant doorway? Does it matter if the doorway is immediately outside the loft (at the head of a set of stairs) or at the bottom of the steps?
I know that for two rooms side by side, separated by an opening (like an arch), a mezuzah is required -- there doesn't actually have to be a door that can be closed. I'm unsure whether a space that "shares" part of another room's space (the loft and the main room share the ceiling) is different somehow.
Here's an example. The steps to the loft are behind that doorway on the left in the back: