This isn't as much a tree house are it is a club house type thing. The picture at the left is the final design. It was originally constructed on "parallel bars" (used for gymnastics) installed by the previous owners. The contruction was failry basic. I used pressure treated plywood and 2x4's for support. My first design was built in late elemtary school through middle school. The first design was 36 inches wide, 8 feet long, 5 feet high inside (at the maximum) and 4.5 feet off the ground. Windows were cut out on each of the long side in the middle. A door was cut in the floor on the far end. I added a ladder to the underside attached with hinges to allow it to be hoisted out of the way for mowing the grass. There was electric lighting and a sink. Starting in high school I increased the size by more than 60%. It required I sink my own ground supports. I also added gutters, window screens, improved lighting, shelving and sealed the inside from bugs. After many years it is still in good shape.