Construction tutorial

The house itself

Walls

I use plywood for the walls because it is available in large sheets which can be cut to size with a jigsaw. It is also very strong due to the alternate direction of the veneers that it is made up of. Plywood does not normally look attractive, so wooden cladding boards may be better. These are simple to fit and cut, but the gaps between each board will close in winter and open in summer, due to the wood swelling in damp weather. This could let rain and wind into the treehouse. You can get round this by overlapping the boards, which also looks good. Boards do not have the same bracing effect as plywood because they can slide against themselves slightly, so you would need a rigid frame to begin with, possibly made of stronger materials.