Planning one now. 16'x16' deck with an 11'x11' cabin. Both for the kid and as a guest house. That way I won't feel bad if he uses it like twice.
I'm also going to build it modular- separate walls screwed together on top of the deck, separate roof trusses and then some translucent poly roof panels. That way I can move it or sell it later if I want
If you're using more than one tree you'll want to use a floating mount on one end at least so the tree sway doesn't fuck shit up. A tree is a looong lever. I'm going to use 3/4" x 12 lags, and probably a j bracket on the float end. Eyebolt with cable and chain is ok as well, some people just slot a hole, and I'm sure someone will sell you some fancy ass stuff if you want. One bolt per end- do not put more than one bolt within a foot or so of another or the tree may 'compartmentalize' that area and it could rot out around your hardware. Good idea to space out your beams from the tree- a short piece of galvi pipe and some washers on your lag works well. Good idea to use a safety backup as well, usually an eyebolt and cable. 14" tree is fine depending on species.
I have friends who saw lumber, so I'll probably have $250 into the deck materials. No idea what retail dimensional would run. Looking like about the same in hardware between lags, forged eye bolts, cable stuff, etc. I have builder friends and the all seem to have a window stash for personal projects they've offered up. Ask around. A treehouse needs windows, and free windows saves $$$ on sheathing. I'm going to use cool old single pane ones and hinge them at the top with a simple leg down bottom to keep them open.
Just some ideas. Post up as you go.
