Foundation selection for terrace

This is how we built our terrace. About 30-40cm of compacted gravel and then on mortar bags or stones. This is already the second terrace in the last 10 years. No point, strip, or any other foundations. This is how it looks finished:

The mother of all terraces. Incredible.

If the planned terrace extends into the clearance area, it is questionable whether you are even allowed to build it

This has already been agreed with the building authority; as long as we stay within 3 m in depth, we could theoretically build a terrace around the house on our property, but it should now only be 3x7m.

Great terrace in the pictures, is the ground a clay soil? I would also like to do it like that, only when it rains here, the ground becomes slippery and soft, I have concerns that there will be problems regarding weight and such, and everyone writes about a frost protection depth of 80 cm, so why only 30-40 cm, have there never been any problems?

The utility lines lie at 80 cm and are only within the last meter of the terrace planning, so not completely in the middle. But I also think it would certainly be advantageous to keep the basic structure minimalist so that in case of any repairs, a heavy foundation is not in the way, which is why I like the terrace in the pictures very much, quickly built, but also quickly taken down, relatively^^
thanks for the many constructive answers

Our soil is somewhat like cake dough. Therefore, we had to excavate to a depth of about 1.50 m and build up about 1.60 m. Our terrace is currently under construction, and as a base for our paving and the path around the house, our earthworks/garden landscaper has laid lava and repeatedly compacted it. Now the curb stones are in place, and hopefully the terrace will be usable in two weeks.