But don't they already do? either cities have strict rules about how buildings look like (rare) or they don't. Probably what you are seeing is just a side effe

But don't they already do? either cities have strict rules about how buildings look like (rare) or they don't. Probably what you are seeing is just a side effect of the slow effect of gentrification. When you have old houses in desirable parts of town, it's often better to level them to ground and build a new one rather than refurbish it. New builds are mostly dominated by contemporary aesthetics because... well, they are what's fashionable now. If this happened in the 90s, you would have a lot of Mc Mansions with 90s aesthetics, and in 20-30 years you'll get houses with whatever style is popular then.