I bypassed codes under an owner/builder exception. No inspections no building plans basically no regulatory costs. I found ultra cheap land with an unproven poorly documented but already drilled well share that turned out to be good (high risk but high reward) so nearly zero for water connection and well. I took some high risks proving electric and water and septic but after almost a year of footwork I proved them -- massively increasing land value for basically free.

Only graded the footing, no excavation under footprint of house, so one day rental backhoe, under $1k for entire dirt work. Poured concrete (300 bags) by hand one bag at a time without concrete truck. Transported blocks and built block crawlspace wall then ran dimensional lumber across, no engineered lumber or piers.

Frame light wood structure to minimum code following irc so no engineering nor architect. Plumbing almost all on single wall with only toilet/tub/sink/sink. All electric appliances, I diy connected to power grid running my own secondary mains.

These last year's prices I think if you exclude utilities I'm at about 70/sqft, you could probably do 50 with reclaimed material and a flatter roof. I double timed after work a couple years but took off maybe 3 months so well under 100k lost wages.