...so basically, my concern is to know whether I can expect a prefab house manufacturer to design a floor plan for me (of course, it would be nice to see the whole thing in 3D) and to advise me comprehensively before I have to sign a preliminary contract?
I think the question has been answered.
However, I don’t quite understand the problem. There are floor plans upon floor plans. The individual house design for yourselves takes time. It’s not just something whipped up out of thin air within an hour. It develops over weeks – and you’re apparently not willing to wait for that either. What comes out in an hour is what already lies in their drawer as a standard model house and was presented to you. That can be
very well used as a basis for calculation. Every home builder offers a 160 sqm city villa, a 120 sqm semi-detached house, a 114 sqm bungalow, and a 140 sqm gable roof house. With standard specification, you can plan well and determine, for example, that Weberhaus is too expensive and Scanhaus Marlow Marlow is too cheap for you. Or that you need an extension for a 5th room. That way, you bring the construction matter to the point... later comes the construction service description, but that is another topic. The gut feeling about the provider also comes into play.
But going deep into the house design is, in my opinion, going too far when nothing is settled yet. This involves work costing a higher four- to five-figure sum, as an argument about a purchase of around €250,000. I think you underestimate this service.
We actually only came across all the prefab house providers because I like timber construction and all the providers advertise how flexible everything is. But actually, we don’t want a house from a catalog. Then for us, the question will really arise whether we might get along better with a regional general contractor.
A regional general contractor will also have their model houses with which they calculate. (As an architect, often only a draftsman is employed who nicely draws everything in his 38-hour week. The architect may work on an hourly fee basis, but only for the GC, and that too hourly. For one house, he has so many hours available... this calculation has to be paid by someone.)
There is the basic execution version with the GC which costs such and such, the better one such and such. The builder sniffs out their financial limit. Both nationwide and regional. The company Heinz von Heiden all over Germany as well as the small one in the north of Hamburg.
And honestly: the performance of the home builder company is not assessed by the design/floor plan/3D visualization but by completely different things (gut feeling, construction service description, good reputation, quality, etc.). A good floor plan can if necessary also be achieved by detours (architect and then GC); it’s not only good because there’s 3D.
What you want is the service of an architect who sees you personally as a client. He can also plan in prefabricated construction and look for a BU who will later implement the house.
Getting quotes has become a kind of popular sport. There are (unfortunately not rarely) people who just want to know what their dream house costs, who have no plot yet (or don’t know what a development plan is and how to read it and whether it is even possible there). Even some who haven’t had a conversation about what loan volume they can manage. Just because a serious interested party “hides” among a handful of dreamers, would you really write everyone a quote?
That’s how it is.
And never misjudge the seller! You can both over- and underestimate them.