The following is a list of step I used in building a tree house or fort at least for this summer. I took upon this endeavor because my son saw tree houses in some of his favorite cartoons. I used the following sources for design and information:
1) Mike Swenson - friend who works with my wife
4) Tree Houses and Play Houses
Select an appropriate location
Mount 2x6 PT Beam to tree. Use 8 in long 3/4 in diameter galvanized lag screw (thanks Nutty Co. in Derby, CT) and secure to tree. Notice the oval cut in the wood. This was done in order to allow for tree movement.
Install 4x4 post. Not surprisingly, I hit roots about 18 in, but I installed it in Quickcrete anyway.
Install two additional 4x4 posts for front support.
Use two 6 in long 1/2 in diameter lag bolts.
Level support beam to the first support beam connected to the tree. Add second support beam.
My helper is installing a pilot to go through both support beams and 4x4.
Install 2x6 outer perimeter joists, square off, install 4 inner 2x6 joists.
Set ten foot 2x6 joists spaced 18 in apart on top of support beams. Use 3 in deck screws and mount front and rear floor joists to the cross joists.
Add another support beam and connect to the front side of the tree and a forth 4x4 post set in Quickcrete. Because I was not able to dig very deep on any post (below 18 in), I installed a 4x4 brace to the front 4x4 posts.
Mounted a 2x4 brace with a 6 in 1/2 lag screw.
Note: Connect floor joists to beams using rafter ties and galvanized nails and all joists were secured with joist hangers.
Constructed the stairs using 2x6 stringers (rails) and treads (steps) and 3 in deck screws. Used 1x4 support cleats nailed to the rails for additional support.
Used standard 5/4 deck boards connected to the floor with 2 1/2 in deck screws.
Added the guard rails using 3/8 in bolts mounted to the floor joists. The balusters were mounted to the 2x4 rails using 2 in galvanized deck screws. The 2x4 rails were mounted guard rail beams and to the each other with 3 in deck screws.
Install a 2x4 rail cap to the guard rail.
Install a guard rail support to the outside of the rail.
Good luck for your own project. Below is list of tools used during the construction.
Two cordless drills/screw drivers