The main comment I have ( and it's not a kook one)is you got to many hall ways. GET RID OF THEM! Your goal is to make use of every inch of space in the box. Hallways are a major waist of space. I always always avoid them. I am a true believer in the "OPEN CONCEPT" design approach and combined use of space. Get more from less. Look & feel Bigger.
Below is another take on the pond cabin/shop. From what you're saying you are on a tight budget. The plan below is based on a 24 x 36 foot print and yields you more shop and living space by combing them together. Once abandoned the kitchen could become the work bench area of the shop.
I have to agree with Eddie. A single story is much more economical than multi story. Have you considered a metal building structure? The cost of the unfinished shell is far less than a stick built home or shop. THey can be finished out to look like a stick structure if desired. The cost of the foundation alone would be far less.
The foundation you have planned would cost a fortune in my neck of the woods. Perhaps due to the soils or frost in your area the large footings and grade beam are required. Good luck my friend.
One last comment "DO NOT USE OSB under the metal roof" period. OSB will not hold wood screws near as well as good old fashion plywood. 5/8 thick min. If you don't believe me just ask my neighbor. His entire metal roof has peeled back over the ridge twice in 10 years. Mine has never even rippled. It cost me 2500$ more than OSB but money well spent. OSB is fine for the side walls. Although I usally put good ole ply wood on the four corners for shear loads.
