We actually considered buying in a development like this – CC&Rs but no HOA. It even had an arch review board.
So before we purchased, I contacted the board and asked a hypothetical question: what would happen if someone built without arch review board permission?
The question was met with a deafening silence.
I took that to mean that, since the review board is staffed with volunteers and there are ZERO funds since there is not an HOA with monthly dues, there is nothing they could do.
An individual homeowner could likely sue the violator, but that homeowner would have to bear all costs associated with the lawsuit.