I'm in a good spot for water. I'mI already had quotes for the septic and well installation. But I suspect the real work will be the foundation. The south is not kind to any concrete, with its soft and perpetually soaked clay.
How big you want it to be will determine what kind of foundation you need for it. I also live in an area with a lot of wet clay, though here we have long heavy snowfall winters and since you said you're in the south I assume you don't have that issue.
The "I never want to fuck with it" option if you don't want to pour a concrete slab is going to be Helical piles, are you willing to rent equipment?
A helical pile is a steel shaft (round or square) with one or more helix-shaped plates welded near the bottom.
Those plates act like threads on a screw. When rotated into the soil using a hydraulic machine, the helix plates pull the pile downward in a controlled, predictable way.
How They’re Installed
A machine (mini excavator or skid steer with a drive head) rotates the pile into the ground. The pile literally screws down until it reaches the required depth and torque (torque = how much resistance = load capacity). Additional shaft extensions can be bolted on to go deeper. Once the correct depth/strength is reached, the top is cut level. A bracket is attached so your cabin beams can sit on it. No digging. No concrete curing. No waiting. An entire cabin foundation can be installed in one day.
However, I do not recommend this as I think it's complete over-kill.
My personal favorite style, and what I would probably use in your situation is concrete footers which you sink below the frost line (which is about 4ft here, obviously if you don't have a frost line, I'd look up local common practice, I know in North Carolina for example, in swampy areas, they typically go down at least 8 ft. Sometimes 12. On top of those footers you're going to put 6x6 treated posts. (I personally would suggest buying pressure treated 6x6s and then re-treating them yourself with used motor oil, while this is illegal, no one will ever know and you will never, ever, have them rot, this is what all farmers used to do to any wood that touched the ground.) If you don't want to treat them like them, then I'd recommend pouring concrete around the posts, (I think this is overkill, it does work though.) On top of those 6x6s you will put adjustable screw jacks (commonly called basement jacks where I live), and on top of the jacks you will put your beams.
The genius of installing these jacks is that if the building begins to fall out of level you can simply adjust the level manually, they are not expensive, and you will only need 4 to 8 depending on the size of the building (one at each corner, or one at each corner and center depending on how large). Then you simply build your sub floor on top of that.
So to recap, I'd suggest:
Concrete footers sunk deep enough to be effective for your local area.
Treated 6x6 posts.
Beams sitting on top of the posts.
Adjustable screw jacks between the beam and post so you can re-level over time
If sunk deep enough to begin with, I doubt you will have to re-level for years or at all, but if you want the thing to last, however, if it does happen, consider how hard it will be to re-level it in 10 years if you have to manually jack up the entire building and them shim/adjust the posts to re-level. The cost/benefit of installing the jacks at the time of building is just insane.
Me and a crew did this with a 30x30 building around 3 years back and there has been no issues with it.
