Experiences and advice on planning a single-family house - advice or feedback?

Prices like that made me turn my back on Munich 25 years ago. Even back then, such outrageous prices were being asked.

The municipality of Ditzingen still offers #27 on the website. Also for 1200/m2

That is the minimum bid in the bidding process. Of course, it might be that no one bids, but I wouldn’t bet on it... 100,000 for the basement in addition compared to the slab? That didn’t really seem that way to me in the first calculations we were given; otherwise, that might be worth considering.

100,000 for the basement additionally compared to the slab?

Whether it becomes 65k or 105k is, in the end, "irrelevant." Irrelevant because you can build a lot of usable living space above ground for that amount. 25-35 sqm more in the house already makes a difference (it can also become a more usable attic. A house with a gable roof is ideal for that.)

This is the minimum bid in the bidding process. Of course, it might be that no one bids, but I wouldn’t bet on it... 100,000 for the basement additionally compared to the slab? That didn’t seem to me to be the case so far in the first calculations we were given, that might be something to consider otherwise.

Including the necessary earthworks and removal of the remaining soil, I consider 100k as additional costs quite realistic.

The supervision effort by the always advisable construction-accompanying expert (the "site manager" never even remotely adequately replaces the site manager without quotation marks, I have written about this many times here).

The complete sentence would continue: "is exactly the same."

100,000 for the basement in addition compared to the foundation slab?

We are further away from Stuttgart, attached is an offer from the civil engineer. The main costs for us are not the labor but the pure disposal of the excavated soil.
Additionally, 5,000€ for stairs, 10,000€ for flooring, you’re already at 55,000€ and "the basement" is not even included yet, the walls are not yet built and plastered, drainage not installed, and no electricity yet. I think 100k€ will almost certainly add up, possibly more.

Whether it will be 65k or 105k, in the end it doesn't "matter". It doesn't matter because you can build a lot of real living space above ground for that. 25-35sqm more on the house already makes a difference (can also become a better usable attic. A house with a gable roof is ideal for that.)

First of all, thanks (also to everyone else) for all the hints, that is absolutely helpful! Unfortunately, a gable roof is not possible, a flat roof or a shed roof up to 8 degrees pitch is prescribed. The cellar was actually one of the first things we agreed on, so it doesn't feel good to discuss it again. But spending money for nothing (or little) is not our goal either, so thanks for the tips, we'll think about it again.