8'6" is max from dirt to roof ridge. So, I'm eyeballing the roof pitch pretty carefully, because at too shallow of a roof, I can no longer use cheap shingle roofing, because it doesn't shed water well enough. It'll be tight getting a workable wall / door / roof overhang height, that I won't hit my head on. :)
Foundation--I think I'm setting skids on piers, and doing a wood floor. The biggest concerns are the softness of the surface soil and the need to accomodate occasionally significant surface water. It'll need *some* elevation. Skids will help with that. It's fairly easy to work with and fairly inexpensive.
Perimeter foundation would offer too much center deflection unless I use ridiculously sized joists.
A concrete slab would distribute weight nicely, and can add height. But it's an annoying proposition to pour a foundation, though because the creek prevents you from gettting any serious equipment back there, so you're humping in the concrete bags and mixing it there...setting up the rebar...then waiting for cure time.
And seeing how soft the soil is, I certainly can't count on simple on-grade foundations; that stuff will just sink right down.
Setting piers down deeper into the earth will keep the structure in place. I will probably be lazy and use the old small concrete slab where my skids run above it, because the idea of busting up all that mesh-reinforced concrete is back breaking, even if you've jack-hammered it to pieces.
So...local code says frost line is 24" down. That'll be the depth of my piers.
I think I'm going to use this "redi-footing" product, actually, for the piers. Again...no concrete sounds appealing to me. I'll still need nearly 3 tons of gravel though, to level out the land and provide drainage. So there'll be plenty of work. :)