You can build a simple tree house out of pressure-treated 2-inch-by-8-inch wood boards and 4-inch-by-4-inch pressure-treated posts. The tree house has an elevated single-room design with four posts to hold it in place.
Begin by securing one of the 2-by-8-inch boards in the space where the trunk of the tree splits into two branches. This board helps to designate where to dig the holes for the posts. Next, secure a second board parallel to the first and brace it in the middle using a clamp and one of the 4-by-4-inch posts. Mark the spaces directly under the ends of the board for the other two post holes.
Dig the post holes, and fill them with cement. While the cement is still wet, place one end of each of the 4-by-4-inch posts into the holes. Make sure that the tops of the posts are level with one another so that the floor of the tree house is level.
Frame the four corners of the square created by the tops of the posts with the 2-by-8-inch boards. Run joists throughout the square space to support the floor boards. Build the walls out of the boards, and then install the floor and roof.