I think the problem is, if these are in a "park" the neighbors will never complain so these "additions" just happen. They do not have public roads inside the fence so you don't really get much of a chance to drive by for a look.
There are a few gated "foo foo" parks with HOAs that will keep a tight reign on the owners but most are just a free for all. The park owner might raise your rent if you add on tho.
