The yard where we live now doesn't have any decent sized trees. None of them are big enough to climb, let alone support a tree house. Since we're also raising two young boys (2.5yrs and 5yrs), something had to be done. After thinking about it for a while, I decided to get some redwood and build an elevated platform NEXT to a tree (so technically it's not a treehouse but it's pretty close!) If I kept it at or under 64 square feet, I wouldn't have to get a permit for it - but it HAD to be big enough for the boys and I to camp out occasionally, so I figured 8'x8' would do it. I made a very rough sketch of the vague plan so I would minimize any mistakes (in years past I would tackle something like this without writing down a thing, and I ALWAYS ended up cutting a critical piece a few inches short!)
Because of the sprinkler pipe I had to move the second set of posts closer than planned to the first two - only four feet apart instead of the five I had planned. It still seemed like it should be sufficiently sturdy, so I wasn't too worried. I nudged the next two posts into position so they were more or less square, then cemented those in as well.
So far the boys have only sustained one injury. Anderson was climbing up or down when I had the aluminum ladder there, and managed to get down to the ground without using his hands. Or his feet. Mostly he just used his head :( But he was none the worse for wear the next day, and it hasn't deterred him at all - hopefully it's just made him a tiny bit more careful on the ladder!
(All the pictures are here)