Housing Interior vs. Exterior

Right now we have all these nice exteriors that we can choose for our houses. I also understand that Square is working on the whole resizing of the interior thing but why cant we change the layout of the interior. I have a small house and instead of just having the one room and a basement, why cant we change the actual floor plan of the house? Like this could be changed when you choose a different exterior. The floor plan could make actually match the shape of the exterior house. I have the cute cottage house and I have a backdoor on my house but its not usable. Why is it there?

You would need to put up walls and doors. :)