Building a tree house in Illinois

i'd check with your city hall.
i would think as long as it's not huge you wouldn't have to,but you never know with some of these towns.

I think you should check with Illinois Council. If you look on their website you will see the telephone number of the building permission. I think it might depend on the height of your tree house. There might be restrictions that you need to become aware of. I have always dreamed on building a tree house. Good luck.