Financing house construction offer 138 sqm single-family house 2 full floors + basement

I can't say anything about the calculation. I don't find the floor plan that bad. A few things: You have quite a bit of hallway on the ground floor, which is due to the square floor plan. I would move the door to the office a bit downward and plan a niche next to the front door at the top for a coat rack. 4 meters width in the living room is not much, but it's doable. Where is the sofa supposed to be? Where is the access to the terrace? You don't want to block the way outside. At the moment, the dining table is also exactly in the way when coming from the kitchen. A kitchen with an island probably won't work if the kitchen also needs to be closed off with sliding doors. In my opinion, the room is too small for that. On the upper floor, I would add a window to the stairs and the dressing room. The bathroom above the living room is inconvenient due to drainage and also causes additional costs. Therefore, swap the bathroom with the northern children's room. It's also nicer if both children's rooms face south. The storage room can then be placed at the end of the hallway if needed.

You actually mention everything that needs to be mentioned

What exactly is included now? House with basement without tiles, painter and floors? WU basement, KfW55, etc. all on top?
For 140 sqm that would be quite cheap, that’s 2500€/sqm including basement

Exactly what is included will be what almost every construction service description states, as also quoted here by the OP. They don’t differ much. Much is not mentioned, or only the standard, which one wouldn’t really want for one’s own house.
Still, at least the interior finishing of the basement is included. And a few small extras. It can also be less in a construction service description.

Without dimensions nothing can be said about the floor plan. It’s okay, it will work.

- You are aware that aerated concrete does not provide the best sound insulation?
- If the upper floor is installed as planned, the bathroom drainage will run right through the living room.
- Poor electrical equipment has already been mentioned. Few sockets, no LAN, electric shutters probably only go up and down without logic.
- The room height in the basement would be too low for me. 2.375m raw construction height - floor buildup. What exactly does that result in? Have you asked already?

Electric roller shutters probably only go up and down without any logic

But that's not really a disaster. The logic of the circuit is only as good as how well you live according to a schedule yourself.

Ceiling height in the basement would be too low for me. 2.375m raw construction height -

I don't find that bad if only one room is converted into an office. A basement is a basement, and this is not intended as a residential basement. Although underfloor heating is mentioned, there is no residential window. You could ask again what you really want there.
But in principle, not every house calculation has to include everything optimal. Not everyone needs the optimum everywhere, whether spatially or in usability. Because then you have to mention every "could be optimized." But then we would be talking about a house that is somewhat larger, somewhat more expensive, and somewhat more comfortable.
For me, from a planning perspective, the trapped dressing room is a no-go, but that is not a construction error you couldn't live with.

If the upper floor is set as indicated, the drainage of the bathroom runs right through the living room.

Yes, that doesn't have to be. It can be done better and more cost-effectively.

This is not really a disaster. The logic of the circuit is only as good as how strictly you follow your schedule yourself.

Yes, it’s not a disaster, it was only meant as a hint. On the other hand, if there is only one switch for manual operation up and down, you might as well just use a strap.

I don’t find it bad if only one room is converted into an office. A basement is a basement, and this is not intended to be a living basement. Although underfloor heating is mentioned, there is no residential window. You could ask again what you really want there.
But in principle, not every house calculation has to include everything optimal. Not everyone needs optimality everywhere, spatially or in usability. Otherwise, you would have to mention every possible “can be optimized.” But then we are talking about a house that would be somewhat larger, somewhat more expensive, and somewhat more comfortable.
For me, from a planning perspective, the trapped dressing room is a no-go, but that is not a construction error that couldn’t be dealt with.

I always find it suboptimal when raw construction heights are given instead of clear room heights. In the end, it will probably be around 2.20–2.25 m. That’s too little for me. I also think the basement should have a height of at least 2.3 m. Of course, everyone has to know for themselves what they want. An acquaintance of mine put a treadmill in his training room in the basement some time ago. He also saved on ceiling height back then and now had to look specifically for a treadmill that builds up as low as possible to even be able to use it in the basement.

If you want to use the basement for residential purposes, then plan at least 2.4 m finished floor / ceiling.