Pole buildings have the least expensive and most stable of foundations. You don't need to bring in the big backhoes and bulldozers. You don't need to hire someone to do masonry. Just dig holes, plant 4x6 pressure-treated posts, and attach your structure to the posts. It's much easier, far less expensive, and more stable than a conventional foundation. A do-it-yourself pole foundation is mostly suitable for a one-story house, though, and it doesn't allow a basement, hence the dominance of conventional foundations.Dig your pole foundation holes below the frost line, but at least 3' deep even in frost-free areas. Make them about 16" in diameter, and make sure they're exactly where you will want your 4x6s to be. The spacing of your posts will be determined by local building codes. In my case, it was a maximum of 10'. If you are using an old mobile home as your interior, you'll want it in place before you start placing posts, and you'll want the posts long enough that you can crawl comfortably underneath the mobile home and between the mobile's roof and the bottom plane of your new roof's trusses. Make sure the mobile home has a firm foundation of its own, set on piers that extend below the frost line, or it will move around inside your new shell.
If you live in an area where the soil contains enough moisture to allow you to pour dry concrete mixture and have it set, pour at least 6" of dry concrete in the bottom of a hole, insert the 4x6 post, make sure it's plumb, and fill the hole with more dry concrete. If you need to pour wet concrete, let the 6" base harden before you insert the 4x6 post. The 4" side of the 4x6 faces the inside of the house. You'll need to brace the 4x6 with some long 2x4s attached to ground stakes while the concrete sets.
This photo shows the posts in place, 10' on center, with half of the roof done.Next: Roof