Backyard Treehouse Is It Legal To Build Something In Your Backyard That Looks Into Other People's Backyards?

Is it legal to build something in your backyard that looks into other people's backyards? - backyard treehouse

The neighbors behind my house has decided to build a tree house next to my fence and must be greater than the barrier, so that children can see my house / garden? I live in Oregon, so I do not know what the laws are built here, but think it should be illegal, something different about someone, I do not d 'privacy in your own garden, and I need help! !