There won't be spam houses
Lots will work as such: you either buy an already built "building"
Or you buy an "empty" lot, that is basically level with the road and looks like a construction yard.
If someone can't build a decent looking building or it doesn't match the city's theme, their lot will instantly be repositioned, they won't be refunded, and if it becomes a repeat offense where they keep making log cabins in the city, then they will progressively receive harsher and harsher bans.
There is no excuse for having an ugly build here because if you're a stuffty builder, there will be pre-made structures and people you could hire to build for you.