Single-family house - Opinions on our design

Unfortunately, I never got to see the original plan. Demolition should be avoided if possible. The foundation slab is not particularly small, and something suitable can definitely be made from it.

Do you know the vehicles that look like a Ferrari but were actually a VW before?

What did your architect actually say about using an unknown foundation slab and risking his head for it in the planning? Or does he have the original plan?

We bought the plot with the existing foundation slab. Therefore, the KG pipes were already in place.

Aaauuuaaa!!!
Now I naturally wonder which further confessions will come in thin slices. After your most recent sample of humor, I had still assumed that the foundation slab at least grew from your builder’s mess, and that a construction interruption had only occurred due to disagreements about changes to the floor plan layout.

A demolition should be avoided if possible. The foundation slab is not particularly small and surely something suitable can be made out of it.

Accordingly, you did not consider the foundation slab to be a legacy burden and at least did not deduct demolition costs from the plot price, but went to the draftsman with the supposed bargain (?)—yet the question

What does the plan look like for which the slab and main pipes were laid—could this perhaps be used as a basis?

must definitely be asked (and positively answered!). Anyone who sells a plot in a hurry—hopefully not because of a botched (or laid at the wrong height) foundation slab—will surely be glad if someone buys their architect costs off them (?).

I would now definitely check all quality aspects of the foundation slab (hopefully there was photo documentation of the construction up to that point); otherwise the warranty exclusions from the architect and the builder could far exceed the "value" of the foundation slab.

Cool. That’s something different. The "Preis ab OK Bodenplatte" takes on a whole new meaning.

Do you have the floor slab layout with all the media?

Do you have the floor plan of the slab with all utilities?

At least through the building authority, one should be able to view the entire set of plans, after all, the slab is subject to approval.