Re: habitable rooms
"Habitable room" is, of necessity, somewhat fuzzy. While size is a major part of the determination, I would not consider it the only factor.
I would also consider what the room is used for, traffic patterns, it's location within the home, original intent, and actual use. Remodelling might effect a change.
I recently worked on a $1.5 Million "tract home." This home had a number of areas that raised the question.
There was a room, maybe 6 ft wide and 20 ft long, that was located under a stair, and contained a sump pump. While I suppose one could put a couple of cots in there....I seriously doubt it would be used for anything but storage. So it's probably not "habitable."
The entry from the garage was into a room, maybe 8' x 10', that contained laundry equipment, and a door on either end. A "mud room," if you will. Not habitable.
Between the master bath and the master bedroom was a room, easily 12' x 12' with no receptacles, one light, and an attic access hatch. The plans called it a "closet." I'm willing to give them the benefit of the doubt.
On the lower lever was a room with direct outdoor access, completely finished and carpeted, and at least 12' x 20, with attached full bath. Yup. Habitable- even if the plans said "storage."
Similar problems accompany defining a "bedroom." In this same house, there are two rooms next to a bedroom, with direct hall access, and the same size as the bedroom. Unlike every bedroom
in the house, these rooms do not have their own bathrooms- though one does have a door leading to one. Neither has a closet- though one has an inset in the wall that sure looks like a closet missing a door! The plans call these rooms "office" and "library." To me, it's a coin toss.
If I were the AHJ, I'd have to accept the plan designations. If I were the electrician, I'd put them on AFCI's anyway. In a few years, when this place is on the market, I can almost guarantee the realtor calling them "bedrooms."