This particular design requires two or three trees (or branches) in close proximity. It was made over the course of several weekends using new, pressure-treated wood for the support structure and floor and an old fence was recycled for the sides. The roof is a camouflage-pattern tarp. It's not weather-proof, but it stays pretty dry inside: a three-season treehouse, but best for summer! It was made with my 4, 6 and 8-year old children in mind, but has been a hit with visitors of all ages.
Note: this treehouse was built in 2009, taken down due to tree growth in 2013, and rebuilt in 2014. I've updated the text to reflect the (minor) changes I made, but there are a mix of old and new photos throughout.
Step 1: Pick your trees
There are definite advantages in using more than one tree for your treehouse - the treehouse can be bigger, and you have to use less bracing. The tree you see here (behind the magnolia!) has a trunk that splits into three at the base, and these trunks splay out somewhat as they grow upwards. At the height of the treehouse - about 9' (2.7 m) off the ground - one pair of trunks are touching, and the other one is about 4' (1.2 m) away. This means the design has been based on one for a closely spaced pair of trees, rather than for a group of three. The tree is a Garry oak, and they don't grow much further north than this (southern Vancouver Island), so they grow pretty slow here. A solid gnarly collection of old trunks, each about 1' in diameter at 9 ft up.
Start by figuring out how high you want the treehouse. 9' is exciting for kids but not scary. You can of course go higher, but you'll have to take more account of tree movement.
Step 2: Get your materials
For this treehouse, I bought:
2 x 8' lengths of 2 x 8 pressure treated (PT) lumber.
6 x 12' lengths of 1 x 6 PT decking material.
6 x 8' lengths of 2 x 6 PT lumber.
3 x 10' lengths of 2 x 4 PT lumber.
3 x 10" long, 3/4" diameter galvanized lag screws and washers
2 x 8" long, 3/4" diameter galvanized lag screw and washer
8 galvanized joist hangers
8 galvanized rafter ties
Nails, deck screws, pulley for 1/4" rope
All this cost about $250 from the nearby Home Depot, except for the lag screws and washers which I got from Fastenal (~$8 each locally despite being more like $12 online). I also had to buy a really long 5/8" spade bit to bore the holes in the tree. The rest of the materials came from a neighbor's old fence: plentiful amounts of cedar boards and 2 x 4 lumber. It was used for the sides, so you can just substitute whatever you'd like to make a railing/walls.
The first photo is of my plan, which was sketched on a cereal box. I read around a lot first, of course: I found David Stiles' books especially easy to follow. The design changed as I was building it - I didn't end up adding the braces drawn in the bottom left elevation, and I built an entrance platform that I hadn't originally planned.
Step 3: Get your tools
The bare minimum of tools:
hammer | saw | level | square | tape measure | adjustable wrench | cordless drill | jigsaw
A ladder is important, but even a stepladder would be OK if you installed a ladder/rope ladder/stairs to the treehouse early in the build.
Useful but not critical power tools:
Miter saw (cutting lumber to length)
Table saw (ripping lumber)
Router (rounding edges)
Step 4: Mount the main supports
Get a strip of light wood and nail one end to one of your trees at a height about 1 ft lower than you want the floor of your treehouse (to save a future concussion, it should also be at least 1 ft higher than your head!). Get it perfectly horizontal with the help of a level and nail the other end to the other tree. Drill 5/8" holes straight into the tree just above the strip of wood. Do the same on the other side of the trees, this time taking the extra precaution of first ensuring the new strip is not only horizontal but also level with the strip on the other side of the tree.
Now, take down the strips and measure the exact distance between the holes. Subtract this distance from 6' (not 8', unless you don't want the entrance platform), halve the remainder, and make a mark this distance away from one end of your 2 x 8. Drill a 3/4" hole in the middle of the board. Make another mark using the between-the-holes measurement . Now drill two 3/4" holes, each 1-2" either side of your mark and both in the middle of the board. Get a jigsaw and make two cuts between the holes to make a 2-4" long slot. Repeat for the other side of the tree. The slot allows the trees to move without tearing your treehouse apart - the more your trees move, the longer the slot ought to be (note that the slots I cut are only about 2" long, but these trees don't move perceptibly at the height of the treehouse, even in a strong wind. If your trees move appreciably, and/or if you're planning to build higher up, use a sliding beam support).
Now, screw your boards to the tree with a wrench. Use washers, and don't bolt hard against the tree. The space you're giving it to grow is the gap between the support and the tree. The longer you want your treehouse to last, the further you should perch the support away - and the more substantial your lag screws ought to be! I know my tree grew only by about 1/4"-1/2" in diameter over 4 years, but most trees grow faster than this. When I rebuilt the treehouse with 3/4" lag bolts, I gave it about 1/2" to grow on either side.
Step 5: Lay out the platform
Because the decking came in 12' long boards, I made the treehouse 6' long. So you need to cut the decking in half, and lay it out. Leave a 1/2" gap between boards for drainage. Cut two of your 2 x 6 boards the same length as the decking, and the other four to the width of your decking less the thickness of two of the 2 x 6's (which will be more like 1 1/2" each).
Step 6: Build the platform
Using 3" deck screws, attach the four 2 x 6's perpendicularly to one of the other 2 x 6's. Make sure they're spaced so that when you put it up in the tree, the perpendicular joists will miss the tree! With someone's help, put the contraption up in the tree, center it, and tie it down.
Screw the other 2 x 6 to the other end of the platform, and check that it is centered and square. For squareness, measure the diagonals and ensure they are the same.
Step 7: Attach platform to supports
Now use the rafter ties to attach your platform to the 2 x 8s that you screwed to the tree (if the angles between joists are not exactly 90°, no problem, just hammer the rafter tie flat against each joist as you're nailing). Add the joist hangers. Use galvanized nails to attach these, not screws.
Step 8: Brace the platform
As it is, the platform will wobble. Add diagonal bracing made of 2 x 4, and use a single long lag screw to attach both of these to the tree. It's easiest to just cut the 45 degree angle in the 2 x 4 first. Use an 8" lag screw here to make up for the fact that you're going through 2 thicknesses of lumber.
Note that I just used one set of braces on the single tree, because the other end had two trees and the wobble seemed insignificant. You'll need two sets for sure if you have just a pair of trees.
Step 9: Hang a pulley
A pulley is great fun for kids, but it's helpful for hauling tools etc. up to the deck. Put one in now, and hang a bucket from it. A hefty climbing carabiner at the end of the rope is perfect.
If you don't have a suitably overhanging trunk or branch, you'll just have to make one. Lag screw a 2 x 4 (or similar) between the two trees well above the deck (cut a long slot to accommodate movement, because you're higher up the tree), and have it protrude far enough to hang the pulley from. Bonus: you'll also have a ridgepole for your roof!
Step 10: Lay the deck
Get up on your platform and screw down the deck. The only tricky thing here is cutting around the trunks. Use sheets of newspaper to make templates so you can cut out the holes accurately. Be sure to leave space for tree growth and movement.
Step 11: Add an entranceway
OK, so far the build has been very conventional - all the books on treehouses will tell you how to get this far. One of my favorite bits was the following minor innovation. The two big supports poke out far past the platform, and you can use one set of these to make a slightly lower level to use as an entry. Make a small deck between the tops of the supports to about 2' out, then build diagonally back to the corner of the treehouse. Add verticals. The pictures tell the story here. I just used offcuts - with any luck you'll be able to do the same.
Some of the offcuts came from cutting the excess 2 x 8 supports off flush at the other end of the treehouse.
Step 12: Railing
I had lots of 2 x 4 from the recycled fence, so I used two 40" lengths at each corner as uprights. I screwed them to each other first then nailed them to the platform. The handrail was also 2 x 4, laid flat, and screwed straight down into the uprights. I mitered the corners, and screwed the handrails to each other through the miter.
Use whatever you like to fill in under the railing - rope, plywood, whatever. Kids probably shouldn't be able to slip through, though. I had lots of nicely weathered cedar boards which I just nailed up. I used strips of 1 x 1 to hold them in place either side under the railing. The only tricky bit was the angled bit leading down to the platform - a bit of trial and error here, because it is tricky to line up the railing with the sides of the platform.
Step 13: Ladder
The plan was to use a rope ladder to get up, but my 4 year old struggled with the transition to the platform, even though he could climb it just fine. So we vetoed it, even after making a pretty nice ladder. I leaned a couple of 2 x 4s against the entryway, cut the angle, and nailed on two thicknesses of cedar board all the way up. The plan was to make a climbing wall, but I was dismayed to find out how much the hardware cost - about $100 for 20-odd hand grips - so I just cut foot/handholds instead. I'd like to say this was free, but I wore out a big spade bit cutting the holes - it got too hot, and bent. This job was easy if a little time consuming to do - mark and drill two big holes, mark a line between them at the bottom and an arc at the top (I used a plastic bucket) and cut out with a jigsaw. This had really rough edges, so I rounded them off with my little router. That worked great, so I went around quite a few other edges on the treehouse with it. Smooth!
Step 14: Roof
I just strung a bungee cord between two hooks I put into the trees at about 7' above the deck, and slung a tarp over. This looked good, but in actual fact it made the roof too low at the sides. I have a compound mitre saw, so I cut four outriggers, screwed them to the uprights, and gave the treehouse roof a decent overhang.
Step 15: Enjoy!
The treehouse is a great little (36 sq. ft.) haven for the kids; they love it and so do all their friends.
Update 2013: I'm happy to report the treehouse is four years old now, it's suffered no damage from windstorms, snow, or tree growth, the trees are healthy, and it still gets lots of use. We've made a few updates over the years, adding a pirate's treasure chest, a swing, a braided climbing rope, a BEWARE sign we use out front at Halloween, and it's well-equipped with Nerf guns & water pistols. It's just big enough for two single air mattresses, so it's fun to sleep out in, too.
Update 2014: Growth got to the point that I took the treehouse down for safety reasons (the tree started to open some joints). I am in the process of updating the instructable to reflect the rebuilding process, which is mostly complete now.