Is the cost estimate for the construction project really realistic?

Well, painted and with all the Xtras....it's quite inexpensive.

The facts are: 140sqm city villa, clinker facade, partial basement (toilet, party cellar, technology), KfW55,

KfW55 compared to KfW70 already makes a difference, and a partial basement often only causes a marginal price difference compared to a full basement. With clinker bricks, full clinker vs. slips, with windows e.g. aluminum vs. plastic, (sanitary) ceramics, electrical: you can really hide a lot of difference there.

Good day,
the cost estimate in our first conversation with the house manufacturer proved to be pure utopia in hindsight. (Fertighaus) About 12,000 euros were estimated for the preparation of the foundation slab, meanwhile we have reached 24,000 euros (Ende offen). It is not a problematic plot, just a flat meadow in the middle of the countryside with good transport connections.
Costs such as the static analysis (2,000€), creation of a crane area (1,000€), and street closure (1,000€) were not mentioned.