House construction in LK Börde/ Harbke city villa

The plot is already available

Then I would build with an architect. No run-of-the-mill city villa (the house can still have two full floors).

We have already been to an architect who shocked us a bit because he wants to apply the same finishing standard for the basement as for a normal floor.

He may and should give you recommendations, but in the end, you call the shots. If you only want a utility basement, then that's how it is. That should be open for discussion.

By the way, I think it makes sense to talk to several architects because it also has to "fit" between the builders and the architect.

We would like to use a building expert from Dekra or TÜV to identify defects immediately and have them fixed if necessary.

Construction supervision and quality control can also be done by the architect. Alternatively, you can have an architect plan the house and then implement the finished planning with a "construction supervisor" (perhaps a civil engineer). Or with a general contractor ("house building company"), but then with your own expert for quality control.

Since the legislator requires an energy standard, I am also very grateful for your experience in the areas of heating, solar, photovoltaics, heat pumps.

Opinions differ on that; I would discuss the topic in a separate thread or first read through the forum. Gas vs. heat pump and controlled residential ventilation yes/no are very regularly debated here. Also, whether KfW xy "is worth it" or not. Every spring, there is a building fair in Magdeburg. There you can also talk to energy consultants (those who can perform KfW calculations, independently). If you don't want to wait for the fair --> Yellow Pages (aka Google), just go there ;-).

Opinions differ on this; I would discuss the topic in a separate thread or first read through the forum.

I can only fully agree with both: on some topics, the same views are repeatedly brought up; and subtopics can quickly become new main topics of a thread, so it is better to actively outsource them early.