You can build a tree house even if you don't have a tree. Create your own custom-made tree complete with artificial limbs, then build your castle in the air. Garnier Limbs and a treated log provide the backbone of your tree house.
Dig a hole 4 feet deep and 2 feet wide. Place the treated log in the hole. Brace the log upright using a 2-by-4.
Check that the log is level and straight by hanging a plumb bob from a spirit level on top of the log. Adjust the bracing until the log is level and straight.
Mix quick-drying concrete in a wheelbarrow according to the package directions. Back-fill the hole around the pole with gravel. Then fill the hole with concrete. Allow the concrete to set 2 to 3 days before you proceed.
Attach Garnier Limbs 8 feet up the pole according to the manufacturer's directions. Attach one limb each running to the north, south, east and west of the center pole. Attach additional limbs between the first four.
Lay plywood sheets across the Garnier Limbs to make your floor. Attach metal straps over each limb to keep the floor from coming loose.
Make prefabricated walls with 2-by-4s, as shown in the photo. Lay five 8-foot 2-by-4s 2 feet apart to make wall joists. Attach two 8-foot 2-by-4s to the tops and bottoms of the row of joists. Lay plywood sheets across the floor joists. Square the corners with the joist edges and check that the floor is level. Drill 1/8-inch pilot holes for screws along each joist, about 6 inches apart. Countersink all screw holes. Screw plywood to the floor joists.
Attach plywood sheets to the outside horizontally to make walls. Leave an opening on one wall at least 4 feet wide for an exit. Attach a ladder to the side of the tree house.
Paint you tree house as desired. If you do not paint it, your tree house will not last as long.