Pressure treated wood is pretty toxic stuff though. And it shouldn't be needed if you design it well. IMO it doesn't make sense to build a structure with wooden parts that are touching the ground in the first place. Of course it's going to rot! If, on the other hand, you make a concrete, cinderblock, stone, etc. foundation around the base, so that wood is not touching the ground, and include an overhanging roof that protects the siding, posts, or any other wooden bits somewhat, it can last a long, long time. It doesn't even have to be pretty unless you want it to, just solid. Our coop was made out of un-treated, unpainted redwood and it survived a tropical climate yearround, plus one category 5 hurricane, before being dissassembled and used to make another coop 20 years later--which is still standing.
People built stuff that stood for generations long before there was pressure treated wood--you just have to build it right...