I recommend that you find some reputable contractors in your area and ask them to come out and provide their opinions and an estimate.

I have no experience, but I'm pretty sure you will need permits to rent this unit. The town may require an inspection. I suggest you hire a pro to make sure everything is done to code. Odds are the whole thing was never built or permitted as a "residence" and probably not to code. (not saying it was poorly done, just not to "code")

"You can tell from its design that the carpenter that built it was planning to make it a livable space at some point"

With the water runoff problems, a wood foundation and a one inch ground clearance, I'd come to a different conclusion about his intent. Or, to a different conclusion about his knowledge and ability.

I'd be interested to hear what happens with this project, and whether the ultimate outcome is lift up or knock down and start from scratch.

A building with rotting foundation is resting on a half inch in twelve hillside and you ask .
"Is lifting this building something I can do with some friends and jacks, or do I need to hire a house moving company?"

If you and your friends know how,I see no reason you couldn't do it. The question is like me asking "my friends and I plan to lease a Gulfstream III and tour the world. Should we hire a pilot or can we fly ourselves. If you decide to hire somone,I suggest a foundation leveling company over a moving company unless you want to relocate the building.

You can tell from its design that the carpenter that built it was planning to make it a livable space at some point, but it's like a project he just never finished.

Perhaps because it wasn't doable where located. I like Klems analogy " should I hire a pilot to fly the plane ". I'm going to steal that one.

By the time you get a real plan, find a mass of hardwood cribbing, find 2 or 4 steel I beams, rent some hydraulic jacks, rent some real machinery to lift the beams in place, cheaper than feeding a dozen men, actually it a long list, if you don't know what's on that list you will be better off hiring a pilot.

Don't forget a few million dollars in liability insurance, crushing a friend under a building can get expensive.

I don't think you need a house moving company, since they focus on moving a house from one place to another. Assuming you want to jack up the house and replace the foundation, and then set it back down, you need a general contractor with experience in jacking up a building in place.

The length and size of the beams required to raise a 25x25 foot house puts it out of the do-it-yourself category. I've seen it done on some of the home shows and it requires heavy equipment to move the beams and get them under the house.

If the underlying concrete footing is solid, it may also be possible to replace the rotting 6x6 beams a section at a time and only raise the building up enough for clearance to get the 6x6's in and out. However, that will not help with getting space for plumbing. It also won't be fun crawling around at foundation level replacing each section of wood.

6" wide seems small for a perimeter foundation so i would think they would be doubled up to give at least 12" width to set the floor joists. 25 feet is also a wide span for a shed floor, so there should also be a beam or supports to hold up the middle, or the joists should be appropriately sized to span that width. Make sure you have fully evaluated how this shed was built.

Bruce