Thickness of concrete slab for 2-storey building

How thick should a reasonable concrete slab be for a 2-story building? The building is not yet planned, but the concrete slab is to be poured on about 50% of the planned area and therefore must already be sufficiently dimensioned. Should 30cm be adequate?

Actually, that is the job of a structural engineer, roughly speaking if there is enough steel in it, 20cm is also sufficient.

Basically, that is the job of a structural engineer, roughly speaking if there is enough steel inside, 20cm is also enough.

A structural engineer will also not say anything definitive before I specify exactly what will be on top, so the motto is better to put in a bit more steel and make it a bit thicker than having to tear it down later.

But you do know that a structural analysis is necessary for a building application, right? If it goes any further, presumably no one will calculate anything for you nor assume any liability.

You do realize that a structural analysis is necessary for a building permit? If it moves forward, probably no one will calculate anything for you, let alone assume liability.

At first, nothing is supposed to be built, just the slab for the next 2 years.... the documents will then go to the structural engineer for review once the building is planned later to check if it fits. An alternative would be to just put something thin down for parking spaces for cars/office containers and tear it down later. I think it’s simply easier to size it sufficiently right away and get it approved later.

Or can structural engineers also calculate a structural plan based on vague information that will hold up later?

At first, nothing is supposed to be built, just the foundation slab for the next 2 years

Without a building permit, you are not allowed to build even a "foundation slab only," so apart from the contradiction in your statement, you might as well have the structural engineer answer the question.

He will then also tell you whether you should do slab foundation, pile foundation, strip footings, or something else.