>floor is tied to the bottom of the house, walls are tied to the outer walls
>need to cope with partition walls for more than one room
>for a second floor, you'll need to abuse flipping partitions, then group them with modular editing so you don't have misalignments. Most people will likely embed them slightly within the walls.
>2k house coins per partition, plus wallpaper tax per partition if you don't want to live in a white commieblock
I might be missing something obvious, but if I'm not, then it's going to be expensive to make a second floor bros. Wallpapers will only make it worse if you want to have a proper "floor".

I think most people who bother with housing are going to group up and split their funds between floors/walls/furnishings/stairs, while the homeowner saves up to buy the expansions. Otherwise, people won't be able to make anything good for a long while.