I asked since I haven't seen any treehouses around here. I found that in the city of Plymouth Minnesota that they specifically do not get involved with treehouses. Typically, anything 10X12 or bigger needs a permit but if you build it in a tree, they won't get involved unless it is (a) an eyesore or (b) a public hazard.
I think that it makes things safe for me since it will be beautiful, sturdy and in a tree in my fenced backyard.
- underclocker, 2007