Is there some context to who designed this and why? That framing doesn't look like anything a carpenter would produce and I'm 99% sure it wouldn't be code compliant in Oregon at least.
It was meant (I guess) as a DIY project and really it's a shed so the process is similar and the 100 square foot print is usually the largest structure you can build (in a lot of jurisdictions) without a permit/inspections/code.
I mean, just because you're technically exempt doesn't mean that building codes don't matter. It's still generally a pretty good starting point for how to build a safe structure that you shouldn't deviate from just for the heck of it.
Plans: http://redcoverstudios.com/MD100/MD100-plans.pdf