Partial basement/lower level

We are about to start building and finishing with research before starting a cordwood and slipform house. I haven't found much on a lower level partially under grade.

We were thinking of a octagon shaped house with loft space but would it be possible to do the larger space above and the smaller space below? How would we support the larger space on a foundation?  

The lower area would walk out to patio area and be below grass level on other side like 5 to 6 ft composed mainly of slip form walls and poured floor. He wants a ridiculous 20 inch wall (sounds too big to me for our area of missouri) I was thinking like 12 to 14 inch would more than support everything.  

We live in a fairly high clay area with a glacial rock drop of large bedrock sheet  that might make digging difficult for a full lower level.

Like this one but the upper level below.

We wondered if this possible as we would have to dig out  to freeze level anyway at 4ft with a rubble trench.

Ideas may exceed budget.
If upstairs is bigger than down stairs large beams are needed across what ever lower floor wall structure you have.
Who is requiring the 20 inch walls, they may know something extra.
A 5 foot drop off on either side may create foundation problems.

By "lower area smaller" I am presuming you intend to have the uphill side of the upper level on grade and the rest over the lower level. If you have a slope, and especially potential bedrock/ledge issues, this sounds reasonable. If you are not planning slab on grade for the upper level, you could let the excavated ground slope from uphill footer level to lower floor level, reducing the foundation retaining wall needed where floors overlap, and giving you tallish crawl space that can be developed as desired for storage, root cellar, etc.

The slipform would be concrete faced/filled with rubble, correct? 20" does sound like massive overkill for a one-story basement wall, especially if there is any reinforcement. Are you anywhere near the New Madrid fault zone? What sizes of stone are you expecting to use for the slipform facing? What thickness of cordewood are you looking at? The foundation doesn't need to be fully as wide as the cordwood; logs can easily cantilever a few inches or so.

I have some design reservations about the plan shown. What direction does your slope face, and how steep is it? The original plan has bedroom windows on only one wall when most of the rooms have double exposure allowing for cross ventilation, and there is no good solar orientation for the window arrangement shown. It only makes sense for a narrow lot where you don't want views of the neighbors. The layout is also rather wasteful of space, giving a lot of square footage to entry and stairwell that is not shared with the actual living spaces. Unless you want a lot of gallery space, I don't see that expansiveness as a benefit. My opinion as an architectural designer is that this plan focuses more on a design statement than on occupant friendliness; perhaps original site conditions affect the details in ways that are not obvious. The general layout, of course, can easily be tweaked as desired for comfort.