Prefabricated house: first architectural planning and then selection of prefab house provider, or the other way around?

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Problems are usually caused by the dimensions. Walls and stairs set a grid that either offers too little or too much room length on the left and right, the width of the house determines the rest. This is how the predetermined rooms above are formed. If an office is also to be a sleeping area, the guest should not have to ghost through the living room to get to the toilet. Otherwise, of course, one can make an office accessible from the living area. The hallway always needs enough closets, technology usually has to suffice for laundry washing and storage space for various items.

In short: Please include all dimensions, otherwise you have no overview of whether a wall is sufficient or not.

We will make the inside nice for ourselves. And try to break up the high walls with a terrace, entrance canopy, and so on.

That’s exactly what I meant by that. I know some houses that surprised me a lot on the inside, maybe precisely because you had preconceived expectations coming from the outside to the inside. As mentioned here several times before, turning the supposed bug into a feature, even if it may not fit exactly here, and ultimately it does come down to the much-quoted "inner values." If you run out of ideas, you can always post it here sometime for a brainstorming session.

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Well done: scribbled by hand, pencil or pen is secondary.
Poorly done: that is a ground floor - one should not start with it; not only under a (pitched roof) attic but also not under an upper floor (straight walls). If you start with the ground floor, fiddle with it until you come close to satisfaction and only then move on to the upper floor or even attic, then the great crying and wailing begins. Laymen only do that if they intend to remain laymen and want to forever wonder by what magic trick the pros do it better. The trick is quite simple: do not derive the upper from the lower, but the less complex from the more complex. So: first plan both floors, then draw the upper floor, then lay tracing paper over it and draw the ground floor. This tip is only for the readers, not for you!

For you the tip is: stay on the conceptual level for now!
That means first setting up the room program (list), then qualifying the room program (approx. room sizes, table). The sum of room sizes in the "city villa" divided by 2 (50:50, in the "one and a half story" about 65:35) shows you the way which "?" rooms must be reassigned in the lists "ground floor" or "upper floor/attic" so that the sums fit. Your total sum should be 130/135 sqm (because you want 160 and add about 20% traffic areas per room on top). If your list at the ground floor comes to 80 and the upper floor/attic to 50 sqm, then you can either shift about 15 sqm from the ground floor upwards and provide an upper floor or shift nothing in the lists and build an attic above. The instead-villa is primarily the darling of marketing victims, the "one and a half story" not necessarily the first "Plan B". The most bloody beginners are recognized by their thread opening "we want to build a city villa" (because "one" does it "that way" without first turning on the brain). The biggest idiots gladly sacrifice their personal living happiness on the altar of wrangling the maximum eaves height from the begrudging development plan (FCK U, building authorities - here live the checkers!).

In my opinion, the ratio between the roof and the building volume is off. It looks as if the house either needs a steeper roof (not allowed), or must cut about half a floor (also not allowed).

Here you are probably talking about your project, .

a Monopoly-house stretched vertically.

Better a house on Schlossallee than a hotel in Turmstraße.