Construction costs single-family house bungalow

What is wrong with the bricks? Ventilation works as a standard air system with an exhaust motor. Not without reason, but inexpensive.

OK but is this ventilation really necessary? I would have gotten several quotes beforehand and ventilation was never planned in any of them? And what do you say about the heating? A Rotex air heat pump with 8kw is supposed to be used, is this sufficient and also appropriate?

No ventilation is a comfort feature, you don't need it with that budget. Rotex is a cheapo and quite loud. I would be careful with the installation location.

OK but is this ventilation absolutely necessary?

Yes.

But it can already be implemented for about €3k.

Maybe someone can still give me information and suggestions for improvement regarding the insulation or the stones?

Because from what I have read, the U-value of the stones is not particularly good.
And according to the building description, the ceiling is also to be insulated with 240 mm thick mineral insulation materials (WLG 0.35).
Is that enough or would it perhaps make sense to get more out of it with manageable additional costs?

Maybe someone can still give me information and suggestions for improvement regarding the insulation or the stones?

Because, as I have read, the U-value of the stones is not very good.
And according to the building description, the ceiling is also to be insulated with 240 mm thick mineral insulation materials (WLG 0.35).
Is that enough or is it perhaps sensible to get more out of it with manageable additional costs?

Every construction company or general contractor has its own building method. At least when it comes to the stones. If you want to build differently, it would be appropriate to switch to another contractor.
The type of building stone and construction method as well as the type of insulation should be the relevant factor when choosing the contractor. The thickness of the insulation can then possibly be adjusted variably to the KfW requirements.