So imagine you have a room like that. You are only using 1/3 of the space in that room most of the time, because most of the time you are not using the higher volume of the room.

 

So what if we just place 2 more floor layers in that room and turn one room into 3 floor rooms? Of course there will be less space for standing, but you can just move on your crouching position and sleep in those rooms, or use computer, or just lie down. Basically you can tripple your living space.