I am completing a garden shed now, 8x12 feet. It is a shed built to house specifications, studs on 16" centers, plywood, over the studs, same Certenteed Siding as what is is covering my house. Gable roof with roof rafters on 16"centers . Ridge vents.
12" eaves. Floor is 2x6 wolmanized planks 1/8 inch apart so I get air circulation floor to ridge vent. No insulation yet, but it stays 10 deg F cooler than outside. Building sits on 12" piers down 42 " and is strapped down.
Any carpenter can build this in 4 days.
You now have a structure that will be good 200 years. You can insulate and finish the walls with underlayment.
My contractor laughs at me for wanting NASA specs.
PM if you want photos.