Since you want to do an earthen floor there’s no way around stone or concrete footers.
An earthen floor requires You to have a solid form on all four sides to backfill.
Those four sides have to be a material that can withstand the elements. i.e., water from rain, insects, contact with earth, etc.
The only material to create those for sides that can handle contact with earth with out degrading is stone or concrete.
You may want to reconsider the earthen floor idea if stone or concrete work is too difficult for you.
Even though it is the most labor intensive, stone is the least expensive cost wise. Concrete is expensive and so is the form work required for it.
An earthen floor is a dreamy idea, but there’s no way to do it without some stonework.
For a 12x12 cabin, simple piers on grade (either pre-cast or poured) with short 4x4 risers and a wood framed, sheeted (insulated beneath) wood framed floor is a more simple solution. And will require much less labor than stone work.
