That is a loaded question that is full of liability. I would say by your post you are outside the perimters of the 'typical standard building code' references, that being said and for your own safety and peace of mind contact a local geo-tech engineer and have him design the footing/slab for you. Remember you and your family are going to be living in this house, you don't want problems.

FWIW, I have built in similar situations and was required to build and pour a 'structural floating slab' 16"x36" perimeter and 11" throughout the balance along with a hectic rebar schedule. Thankfully the lot was cheap enough to justify this added expense.

First of all find out from your building official what soil bearing value they say to use for your area. 1500 psf is the generally used minimum, but it may well be 2000 psf.

Then you need to calculate the weight of the walls based on plf, pounds per lineal foot. The mfr's data will tell you what the weight of the wall will be on a square foot of wall area. Add to that the weight of the interior and exterior finish. You should be able to find this from mfrs' data for the faux stone you plan to use. I found data somewhere about wall weights on the web. It was actual data derived by weighing wall materials after the walls had been demolished.

Add to that the dead weight and live load of the floor and interior walls supported by the outside walls. If you have interior bearing walls on footers, the load has to be divided up between the outside and inside footers. Next add the dead load of the ceilings, plus any live load required by code if you use attic for light storage. Last, add the dead load and snow load of the roof, factored to plf on the walls.

The IRC has has tables for various loads to use for various types of structures, or you can calculate it all yourself accounting for the weight of all the lumber, sheet rock, flooring, siding, etc., plus some safety. I think you'll find the IRC numbers are fairly conservative, especially the live load numbers. You'll see that live loads for sleeping areas are only 30 psf while for the rest of the house it's 40 psf.

If you plan to have extraordinary heavy items in the house you'll want to design the footers and joists to account for that. I'm adding footers and pony walls under the bath and utility rooms to help support the weight of tile walls and floors and tubs of water. This way I can keep the joists all the same size and have the rigidity I need for tile.

It's not hard, but it is tedious. Then you can come up with an estimate as to how many pounds per foot the house puts on the footer. Take that number and divide by the soil bearing psf and you'll come up with the footer width. What you will find is that there is a fair amount of difference among various parts of the house, depending on much floor area hangs on the walls, the width of roof carried by the wall, the varying height of the walls, and interior building material. If you find a tremendous difference between the weights in one section of wall vs. another section, like maybe a 2 to 1 ratio, you may want to have a civil engineer design your footers. This is because if you make the footers the same width all around you could be well oversized in some sections if you size for the heaviest load, or be undersized in the heavy areas if you size according to lighter loads. Oversizing will, of course, cost more concrete and money.

What's your snow load? Must be pretty good if you've got 30" frost depth. For my house in the bedroom section, I used 30 psf live, 15 psf dead, floor load, 28 ft floor width with center support, 14' icf wall height, 6" flat forms, 20 psf snow load, 15 psf roof dead load, and 20 psf attic load in center third of trusses. The load on the footer calculated to be around 2100 to 2200 plf. For 1500 psf soil that's just under 18" wide footer. I'm using 18" wide by 12" deep all around. I just at my spreadsheet. The heaviest part of the house is the living room area even though the wall is 16" less height. 40 psf vs 30 psf floor live load plus a 24" additional roof overhang makes a difference.

Don't forget to look at how much roof load there is on door and window lintels, and the depth of lintels. This is particularly an issue if you have wide windows, a heavy roof, and not much depth in the lintel.

Also, you may need to look at extra footing area if you have point loads resting on the wall. This would be the case if you have a long girder truss supporting other roof trusses over a large open span. Look at your floor plan and visualize where the weight of roofs and other structures get transmitted down to the ground. You might have a surprise or two!

Hope this helps a little! Now after all this, if your structural elements neatly fall into the parameters of the code tables, save yourself all the headache of the above and use the code tables!

Happy New Year!

Chris, point taken re: liability potential.  I talked to a structural engineer who said only 1-2% of soils tests in our area showed less than 2000psf.  When less than 2000psf, they were fill site or previous construction.  So he says for shorter walls use 16" and taller walls bump up to 20".  Didn't have site specific soil analysis from NRCS.  Will go back to him with that info and also tt geotech outfit.  Thanks for input.

DMACELD. appreciate the primer on determining loads.  Actually very simiar structural details, (2' overhang, 14' walls).  Already id'd point load for interior stone clad arch.  Snow load is 30psf and wind design is 90 mph, but this is southern Kansas and will do more than min rebar for walls.  House will be in country and in cat C exposure (think wheat field).  Your input put the process into a quite logical approach and was very helpful.

Will update after talking to w/engr & geotech.   BTW steel dist'rs say prices increasing $75/ton over next 3 mos or so.  Maybe old news to some.
Happy New to all!

I find it hard to believe that 37% silt/ 38% sand is only 1,500psf, unless you're in a marsh. And you don't say whether you're in a seismic zone.

That aside, what's wrong with designing it conservatively? It doesn't take that much more steel and concrete proportionately, so why not eliminate all doubt?

Quantum,  The site is not in a marsh or seismic zone, so your words are encouraging, in that, perhaps the soils aren't the equivilant of quicksand.  This is well-drained upland farm ground in southern KS.   My nature tends to the conservative and since my wife and I will live here for the duration want to ensure the foundation serves us well for the duration and beyond.  To that end, I started pricing local steel and found grade 60 #4 is only about 20 cents more per 20' stick than Grade 40.  One supplier prices them the same!!  Grade 60 it is.  Will use similar approach for walls, slab and roof.  Thanks for your input.

In case anyone's curious, I got in touch with a local soils engineer to get his take on the load bearing capability for my soil description, CL. He said even though the soil could very well be 2000psf, I'd be safe with footings sized for 1500 psf. He used an interesting example. Go to a sandy beach and walk (1) in the water (not on) and your feet work into the sand or (2) on the damp beach (optimum moisture and compaction) and your feet are better supported or (3) on the dry dunes and your feet sink again. Lesson...to achieve desired compaction and strength your need the right amount of moisture. Now if I could get the rain to hold off until ..... Dave