As far as I'm aware, you can stack big wooden crates up to 3 times which when you think about it, is the normal size of a room if I remember correctly. So what if 3 crates on top of each other would serve as flooring on the second floor? I think it's realistic and a great new way to
move from floor to floor. it will also allow the person to be able to climb to second floors without the need of carpentry level 6 so they can start making their second floors before tediously farming for carpentry Lv.6