Might look at a website like this.
https://jamaicacottageshop.com/shop/20x30-cabin-2/
These timber construction Cabin plans come pre-cut and ready to assemble. I looked at this and it did not seem right for me. Timber construction might be overkill for a simple 1 story shed-like building.
I noticed you can buy them without siding material rather unfinished. These are not cheap but they have some pricing.
Might watch some of these shows:
https://www.hulu.com/tiny-house-hunters
If you are going to live in a building, you need a set of plans, water lines, electrical lines and sewer lines in most residential areas. They do make composting toilets as an option ??? Living in a building also requires some windows and air flow.