Cousin asked for something like this already his idea was basically the same except he said this.

Keep appearance of the house since the plots are made for small, medium and large.
When you upgrade only the inside changes, so when you go into say a small house and you upgraded the inside to medium you will have a medium house inside.

Though this means they would have to give large something since they are already the best house maybe add a cellar area. Though nobody from small or medium can have a cellar.