Positioning of a single-family house on an elongated north-south plot

There is no development plan. And as we were informed by the building authority, we can plan 125 sqm of house area.

So 125 sqm + additional areas such as terrace, garage, etc

Yes

About the floor plan
Create a room program
not just kitchen bathroom bedroom
but
bedroom for bed 2.2x2.4 m, 4 m linear wardrobe etc.
very important is what sets you apart from others. e.g. sewing room, billiard table, shoe collection, 6m floor-to-ceiling bookshelf.
what is important to you peninsula in the kitchen
dining table 2.5x1 m
lift-and-slide door
etc.

By the way, browse catalogs, the internet. Floor plan you like, fits your room program. Fits your daily routine, what needs to be changed
etc.

Floor plans with 3 children's rooms for example
Schwörerhaus E20-165.6
Hanse Haus Variant 35 184

Thank you for the tip. We are already diligently researching floor plans on the internet.