Concrete slab on panel stone wall

Hello everyone. I built a wooden terrace last year. Now I would like to replace it with something more durable because I am considering putting a conservatory on it. Conditions are: about 70 cm high, 6m wide, slab thickness 16cm. I consulted a building materials dealer today and he approved it but suggested filling the huge hollow space in the middle with gravel and using formwork to pour a concrete slab. I would do that but I have no direct access to the garden and have to go through the entire hallway. He also told me today that I do not have to dig 80cm deep for the foundation since the concrete slab has no contact with the soil. His proposed 30cm is sufficient (as long as you are below the topsoil). Am I overlooking something important or simpler because despite extensive online research I have not found anyone doing something similar even though I am sure many people live on raised ground floors.

Plan the conservatory first, then look further. The conservatory has certain widths or post distances. Concrete through the house is hopefully not meant seriously. Will it work with a pump?

So, just to summarize again... you have an apartment on the mezzanine floor in front of which a wooden terrace floats above the ground. It rests on a concrete block wall. Now you want to completely fill the area with gravel and pour a concrete slab? Do you maybe have some photos showing how it currently looks?

I don't see how you could simply transport the necessary amount of material through the apartment. Neither the gravel nor the concrete.

Plan the conservatory first, then look further. The conservatory has certain widths or post distances. Concrete through the house is hopefully not meant seriously. Does it work with a pump?

Are there standard dimensions for conservatories, for example 4x4m? Or something like a universal foundation plan for a variety of conservatories?