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

No, that's fine, I've already checked. No approval required.

We had 4x4 pushed and compacted. After that, the landscape gardener put [SchotSchotter] and concrete slabs on it. Now we can’t find the garden house :D if we do, then it will go on there like that.

We had a 4x4 area excavated and compacted. After that, the landscape gardener put gravel and concrete slabs on it. Now we can't find a garden shed :D if we do, it will go on there like that.

Do you just place the house on it then? Or do you somehow secure it to the slabs? What kind of concrete slabs did you use?
And if I understood correctly, the landscape gardener did all of that for you. What did you pay?

Garden house or guest house?

I cannot imagine that a guest house (living space) is simply exempt from approval.

Interestingly, I don't really know that yet because he "participated" and we don't have a final invoice yet. These are just simple gray concrete slabs - about 40x40 (?). I would just put it on there like that.

First the garden house, later then. But where there is no plaintiff, there is no judge. Neighbor also agrees. If it should ever be checked and there are simply no devices inside, I just have bad luck, take it down, get a building permit and put it up again. For the garden house though, it's ok.