Two important thoughts I had before building the treehouse were to use as much 'waste' materials as I could reasonably find, and also to generate as little waste as possible during construction. The photograph attached here depicts the total waste from producing the studs for one gable end waist wall, two side waist walls, as well as an OSB ridge beam and a pair of rafters. The two stacks of short pieces are the wall studs, the rest of the wood will be top and bottom plates, and two rafters are pictured. All the waste is the small pile of scraps beside the Makita circular saw.
The decisions to include (or exclude) certain features, or do things in a particular manner, are often quite dependant on the materials I have to work with. I was able to pick up a number of scrap 2 x 4's in 48" lengths, which gave me the idea to build an overhanging gable ended dormer with two foot tall waist walls. I simply cut the 4' sticks down to 23-3/4" and added a top and bottom plate, with a sliver of wood as waste per stud.