Floor plan design for a gable roof house (knee wall 2.20m) approximately 170 sqm

Nice that you are still here :) Then just use that as a starting point. At least it is buildable. Viebrockhaus houses work. They are also available as show houses to experience firsthand. In your design, you planned the staircase against the room orientation. That would never have worked.

and there the floor plan of Maxime 700II would correspond to our ideas.

Great! That is already a good first step: you have now identified a catalog house that sufficiently resembles your "individual design" to use it instead. Now continue on this path to success! Step 2: now look for builders in your region. Because a) you are not building near Viebrockhaus and b) I also see a not insignificant probability that for cost reasons you do not want to end up with them, their well-designed building proposal will not help you much with another contractor. Step 3: look for suitable floor plans regardless of staircase type. You preferred a two-flight straight staircase and had already considered a one-flight straight staircase. With this, the chances for appealing floor plans are already well positioned. Don’t just look at "city villas" and designs with high knee walls: with the possibility of two full stories, you are in a comfortable position to also use one-and-a-half-story designs as a basis. Turning a "country house" into a "city villa" is structurally unproblematic (the opposite would not go well), it just results in somewhat larger rooms on the upper floor than in the "original."

Because you a) are not building near Viebrockhaus and I b) also see a not insignificant probability that you do not want to end up with them for price reasons

Viebrockhaus would even be an option and gives a base price of €369,950 on the homepage as a price offer. This is even significantly cheaper than the first offer (for a similar town villa with 160sqm) from a local builder. It is clear that the additional costs for upgrading, incidental construction costs, and utility connections come on top in both cases. The offer from the local builder is €397,900 with little included according to the construction service description. Thanks for the hints, that’s exactly how we will proceed and also consider awarding the contract on our own.

and also consider awarding it independently.

Better leave that alone, it carries too many risks for "first-time" builders – not least the risk that it will backfire financially. Tendering is a minefield for beginners. In gymnastics, mats are good, but nail boards are bad ;-)

Better leave it alone, it poses too many risks for "first-time" builders – not least the risk that it will backfire financially. Tendering is a minefield for beginners. In gymnastics, mats are good, but nail boards are bad ;-)

I understand and totally agree with you, however, my father-in-law owns a heating/plumbing company and believes he can coordinate well with friendly companies from other trades. I think, though, we won’t go in that direction, and if we do, only with the construction manager and architect.

I don't think we will go in this direction, and if so, only with a site manager and architect.

Yes, better to have the tender with the architect - who in my opinion should also manage the construction.