Ah, a cobber! I went to a workshop in Coquille in July 2007, with Ianto and Linda. Same one you went to? There was a young man there taking the class who had built a coop out of cob before the class. I don't see why you couldn't if you have the materials available and the time to invest. I would think an 18-inch wall would suffice. What kind of roof are you thinking? natural, sod and living? That would be cool. As for covering the walls, do you have the Earthen plaster book? There is a mixture, I think using vegetable oil with it, that makes it waterproof. It is what Linda demonstrated to use in the kitchen in one of the houses when I was there. Why don't you PM me?