You can't have buildings which get bigger, as they grew they would overlap any buildings next to them or anything else for that matter.
As for the idea that you level up by adding another building we have two issues. Firstly they have not designed the graphics for that, though that is a easy fix. Secondly and more importantly they would have to think about how many levels of the building a player could achieve considering there is nothing to prevent a player from owning several hundred of these buildings.