Base slab vs strip foundation

So there is always a slab.

(Strip) footings are needed when it is statically necessary/sensible. This is, for example, when the slab would have to become unnecessarily thick for load-bearing capacity or the ground would not be load-bearing enough.

Example from our construction project:
The ground was not load-bearing enough up to about 1.40m below the planned bottom edge of the slab.
Recommendation according to the soil report: excavate the soil completely and fill it in layers with compacted gravel.
Alternatively, strip footings according to the soil report/excavation/structural engineer under all walls.
Although quite a bit of concrete was used for the strip footings, this alternative was about 4,000 euros cheaper.

Edit: The planned slab was of course placed on the strip footings.

Because you can't then build frost-free with just a slab foundation...

But I guess it also depends on what the slope looks like where the house is located and how deep you fill with gravel, right? We built on a slope and only have a slab foundation...

You are funny - if you build with gravel, you don’t need a strip foundation. I assume in this question a soil that is sufficiently load-bearing.

There is a certain layer of gravel anyway, right? The load-bearing soil was just 20 - 30cm deeper than the house should be, and that was filled with gravel and the floor slab was placed on top of it...

No, one does not generally have that. One should fill with gravel if one wants to have load-bearing, natural soil - except on a slope because of frost safety?

I don't know. Every offer we had mentioned something about 20cm of gravel under the slab...