Which prefabricated house provider - prices, experiences?

But then the question is how the house on parcel 73 was ever approved?

Probably old stock from a time when the 3-meter setback rule did not yet exist. In my home village in Bavaria, there are also numerous old buildings that would not have been even remotely feasible by today’s standards: dirty corners of 70 or 120 cm between two houses, houses meeting at a corner, built directly on the street without a sidewalk, etc. Back when the feudal lords were still called "Elector of Mainz," "Prince-Bishop of Würzburg," "King of Bavaria," or "German Emperor," such things were definitely possible.

And just because the feudal lord changed, old buildings have never had to be torn down (except in special cases). The new building regulations only apply to new constructions or possibly renovations. With exceptions for fire protection and possibly environmental law.

Filthy corners of 70 or 120 cm between two houses, houses meeting at a corner, built directly onto the street without a sidewalk, etc. In times when the feudal lords were still called "Elector of Mainz," "Prince-Bishop of Würzburg," "King of Bavaria," or "German Emperor," such things were still quite common.

The filthy corners are probably to be taken literally and have their origin in little shit alleys used for emptying chamber pots. The ancestor of today's development plans or design statutes were setback line ordinances, which at least aimed to curb the fraying of houses built sometimes with and sometimes without front gardens onto the street.

Then the question is how the house on parcel 73 was ever approved? Or is it actually 3m and the plan is wrong?

That's exactly what we still need to find out. But I think the house is simply very old and was built under different regulations.

I'm digging up this thread again and hoping for constructive answers

We have visited a few providers by now, and one in particular caught our attention. Unfortunately, it is one of the rather more expensive ones and we don't know if we can afford it financially. The responsible engineer has offered to provide a rough cost estimate based on a floor plan as we might imagine it.
So we thought it over and planned, and the floor plans below emerged.
The external dimensions are 7.20 x 12.79, the staircase is 2.42 wide.

Is it possible to build on this or are there any major flaws?
The carport is to be built between the front door and sofa, which is why there are no windows there.

Prisoner dressing room and G-WC without daylight I would reconsider.

I would reconsider the enclosed dressing room and guest WC without daylight.

Thank you for the feedback!
The guest WC would take space away from the office in the alternative. Not a good trade-off for us. We also have an internal guest WC in our current apartment, which has never bothered us.
The dressing room is actually not really intended as a dressing room, but rather as additional storage space.