Here is an idea to look at. http://www.small-cabin.com/forum/2_599_0.html
A friend and I actually built a small backyard shed using the above idea. Kind of like playing with a giant Lincoln Log set. He was in rent house, and wanted something that he could use and move when he moved. Used 8 foot treated lumber, gave him about 7x7 inside dimensions. Now that he has bought a house, it has turned into his granddaughters playhouse. .

That forum has some pretty good and what I call crazy ideas that will work. Well worth spending some time getting ideas.
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