Foundation for garden house with approx. 364 cm x 304 cm external dimensions

Garden house or guest house?

asked that because you wrote earlier

later then as a guest house.

So it is purely about a garden shed/storage shed (do you have to differentiate between those?).

I believe there is a court ruling that says municipalities are allowed to levy taxes for a second residence if the garden house "can be used as a second residence due to its furnishings." Water, electricity, and a toilet are sufficient for this. This also applies if the garden house is on the same property.

Back to the original question.
3x4 or 4x4 meters is not huge. I dug out 30 cm and set edge stones. Then 25 cm of gravel (compacted), and on top of that basalt grit. Here I then placed 15 slabs, on which the foundation beams of the garden house rest.
I concreted the foundation beams at the four corners with ground anchors for safety, so the house cannot be blown away.

It was a lot of work. But if you can get gravel and grit directly from the quarry, it's an inexpensive thing. I think I paid well under 50 euros for 4 tons of gravel and one ton of grit.