I cant see how that bathroom is feasible. 3x3 as drawn is an airplane bathroom and you eat another ~8" of space with the framing and drywall (so you're either 28" square or 44" square). I dont even know if building code allows it to be that small. If you gotta have a bathroom (it adds the need for plumbing which is otherwise unnecessary), then stick it in a corner, not in the middle of the layout. Both for the above mentioned reasons and also for re-sale. No one will understand why there's a bathroom in the middle of a building that small.
Honestly, in a 12x24, I think I'd run back across the driveway.
