Is the house height specified for you? For us, the additional thickness of the concrete would have negatively affected the house width (setback areas to the boundary height/2).
Well, the 3k€ can also quickly become more. You have correctly recognized, it depends entirely on the structural engineer. In our estimate, 2.5 tons of reinforcement were indicated. According to the structural engineer, it was 6.5 tons. Steel is very expensive. I would possibly first talk to the structural engineer and then discuss whether alternatively walls should be moved.
Thank you for the feedback. Yesterday, the current plans were sent to the structural engineer, and tomorrow we will probably talk to him. Let's see what he says.