Well, here’s how I’m building one anyway. Like most Dads, I’ve been asked to build a treehouse. We don’t have an ideal tree for this, but here’s what we did:
This spring we cut down a half-dead non-bearing fruit tree. It was hanging over the neighbor’s fence and had to go. But I left a stump about 9′ tall in case we wanted to do something crafty with it, such as build a treehouse.
So this is a treehouse built around a tree but not in it. After getting my kids’ input, I drew up a sketch that was basically an 8′ square platform with railings, about 8′ off the ground, and roof-less for stargazing. I decided to drop four 4″X4″ posts in the ground and build a deck-like structure about 8′ up, leaving about two feet of the tree sticking out of the floor and room for 3′ railings all around.
I used an elaborate set of stakes and mason-string to plot my four holes, placing them in a way that 8′ boards would fit around the outside and of course squaring them up as close as possible (for a treehouse). Of course I had help all the way on this part:

So at the end of the first day, here’s what we had: four gigantic (4X4x12) posts in 2′ holes, more or less level and square, staked up and ready for concrete. I had enough concrete to set two and a half of the posts the first night. The next day I went for more supplies and finished the concreting. To establish a firm footing around the poles, I poured in a layer of concrete mix (not soupy, but muddy), added a layer of gravel from the driveway, tamped all that down good (my trusty helpers did a lot of tamping), and repeating until the hole is full. That way you are using the hardpacked gravel and concrete to support the holes, not just the concrete. This builds strength and reduces the likelihood of cracking.
The plans became more elaborate at this point. We debated over putting in a lower deck for the younger kids. Then we decided to just put in a ground level deck (or about a foot off the ground), that younger kids could play on and adults could sit and sip a cold beverage. So now we’re building not one level but two.
OK, the next day it was time to establish a level line on all four posts for the first floor, and I was thankful for the summer I spent in Greensboro working for Paradise Decking. Those boys sure know how to build decks, so if you live in Greensboro, North Carolina, and need a deck built, call them up. They can build you a huge one and it will be solid.
So anyway, for nine bucks I bought these plastic gizmos that you put on each end of a garden hose (or “hose pipe,” as we call it in the Southern U.S.). Fill up your hose with water and you have a water level. The Pros use a long clear plastic hose, but do you know how much clear plastic tubing is at the General Supply? About a buck a foot, and you need at least twenty feet. So this gizmo did the trick, established our level lines on the four posts, but it was not easy. You just have to be patient. Did I mention that we’ve been under an excessive heat advisory so we try to work early in the morning and later in the evening, but man it’s hot and humid any time.
OK, with our level points determined, it was time to build the lower level deck. I used all the scrap wood I could find while maintaining some level of aesthetic quality. I used two 2X10s and two 2X6s for the outside boards (don’t know what you call them). For floor joists I used pressure treated 2X4s. I plan to do the top right with 2X8s for the outside boards and 2X6s for the floor joists.
Oh, don’t forget the carriage bolts for the corners. I used one for each board, which meant two on each corner, totaling 8. For bigger decks, we alway put two on the end of each board (4 in each post), but I think one per board is plenty for this project. Here’s my helper installing a carriage bolt:

So at this point I was out of lumber and it was time to buy the decking boards and the rest of it that would be needed. Here’s where we are now, first floor almost completely decked:

That’s a honeysuckle bush held up with a bungee chord. I ‘m going to see if she’ll survive. Doesn’t the lower deck look like a great place for a cup of coffee? Well, not in this 100 degree heat but come Fall, this will be a nice sanctuary.
OK, now I just have to build another deck like this, only about 7′ above the first floor. That won’t be hard at all. BTW, have you checked the cost of lumber lately? Here’s my first lumber purchase on the left and my second on the right.

Those 4X4s were 15 bucks at Lowe’s. And you know decking ain’t cheap. Also there is the cost of decking screws and other hardware, the new drill bits you’ll need for the carriage bolts, that new sawblade you’ll need to replace the old one, and of course the brand new professional tape measure you’ll need that is suitable for such work. The 12′ one in the drawer that no longer locks just won’t cut it. But remember, this is just the SirMuddyKnees way. When I do something, I like to do it with quality. That goes for the tools, the materials, and the work itself. It’s just better that way. Read Pirsig’s Zen and the Art of Motorcycle Maintenance for the reasons why this is so. It’s all about Quality.
All for now. I’ll keep you “posted.” Get it?