Foam doing the job that depth normally does
A conventional footing is buried deep for one reason: to get its bearing below the ground that freezes and swells in winter. Freeze the dirt under a footing and it lifts, and it never lifts evenly. One corner goes up a little each winter and comes back down each spring, and enough of those cycles later the block is cracked, a door has stopped closing, and the floor is off. That is frost heave, and depth is the ordinary way to stay out of its reach.
A shallow design takes the other road. The ground itself holds heat, and a heated building keeps feeding heat into the ground under it. Rigid foam board at the turndown edge, and more foam laid out horizontally into the surrounding soil, slow that heat from escaping fast enough that the ground right under the bearing surface never gets down to freezing. The freeze front goes around the insulated block of soil instead of through it. The footing gets to stay near the surface because the cold cannot get to it.
That is the whole idea, and it is real engineering rather than a shortcut somebody invented. It is also completely dependent on the insulation being where the design says it is, in one piece, for the life of the building.
Against a footing or a frost wall carried below the freeze
The conventional route digs. A footing goes in at the depth local code sets for your address, a frost wall stands on it, and the building lands on that. It is more excavation, more concrete, more forming, and often two pours on two days. In return it works the same whether the building is heated or sitting empty and cold all winter, every inspector in the region already knows how to look at it, and there is nothing in it that a backfill crew can wreck by accident.
The shallow route digs much less. Less spoil to move, less concrete in the ground, the slab on grade and the bearing edge going in as one pour, and a finished floor close to the outside grade. The money moves off the excavator and onto the foam, the detailing and the care it takes to build it right. It also narrows what the building can be, because the heated version depends on the building being heated.
Neither one is the better answer in the abstract. What actually decides it around here is what is on your drawings and what your building official will accept, and on ordinary residential work in these mountains the footing carried below the freeze is still the normal answer by a wide margin.
The ways a shallow design goes wrong years later
Every failure of this detail is the same failure: the insulation stopped doing its job somewhere. A backhoe bucket tears a board during backfill and nobody sees it. The horizontal wing gets cut short at an outside corner because the trench was awkward, and the corner is exactly where the ground loses heat in two directions at once. Water pools on top of the foam and moves heat out of the ground faster than dry soil would. An attached space gets built unheated after the fact, over an edge detailed on the assumption that it would be warm.
None of that shows up on pour day. It shows up as one corner of the building rising in February a few winters later, and by then the fix means digging up the very thing that was supposed to save the digging. That is the risk profile you are accepting, and it is why the detail has to be built exactly as it is drawn rather than approximately.
What we would tell you on the phone about one
We pour what the drawings call for. If your designer has specified a frost protected shallow foundation, bring us the detail and we will tell you plainly what we need in order to build it: the foam thickness and layout, where the wings run and how far, how the corners are handled, and who is protecting the boards between our pour and the backfill.
What we will not do is substitute a shallow design for a footing carried below the freeze because it looks like it would save you a day with the excavator. That call belongs to whoever designed the building and to the building official reviewing it. Work that holds a structure up needs a permit and a footing inspection before concrete goes in, so the detail is going to get looked at either way, and it is far better for that conversation to happen over drawings than over an open trench.
If you are still deciding, call and describe the building, how it will be heated, and how much the site falls. We will tell you what we see, and the depth that applies to your address goes on your free written estimate before anybody digs.