Bavaria: 2 single-family houses or a duplex?

Clarify the loan amount, how much you would get, look for a general contractor or architect. Something very rough like that.

I am currently clarifying the credit limit. Do you look for a general contractor or architect before you look for a prefabricated house builder or a house from a specific provider? Or am I mixing up terms vaguely here?

Then the calculation doesn't work. The old stock still has to be removed. How big is it?

Yes, I count that as part of the land costs. I calculate with 50K €. That already adds up to quite a bit...

Do you look for a general contractor or architect before you look for a prefabricated house builder or a house from a specific provider? Or am I mixing up terms here?

Sorry, I didn’t read that it was already decided that it should be a "prefabricated house." You probably mean a standard house made of timber frame construction from a respective company?! Of course, you can go directly to a company. It is common to simply go to a prefabricated house world, park, or exhibition and inquire about quality and price, but also about time frames. Have each company provide a construction service description, the basement company and price, and have them roughly scribble the price for approximately 170 sqm in the upper right corner. Then your homework comes: compare the construction service descriptions. What is included where and how? What determines the price? Who meets their own expectations?

Yes, exactly. It is a plot with old buildings for which there actually is no development plan. I am trying to inquire at the building authority about the floor area ratio and site coverage ratio, but you can see from the surrounding densified plots that we are still mildly sized with 2 single-family houses. The plot is about 31m x 38.5m large. It is already somewhat outside of Munich, just under 40 km.
[...] - Basement: 100T €

Without a development plan (however, you mention lively densification all around, where usually one is established) the building authority can usually tell you nothing about the site coverage ratio and floor area ratio, because these are generally not calculated for development according to §34 (but are first calculated for someone who clearly wants to exceed the scale). Given the size of the plot, I would find the idea tempting to perhaps mark out a third building plot (e.g. about 420 sqm each for you, and about 350 sqm for a third party). Regarding basement costs, I refer you to my basement rule (here in the forum, explained several times recently in these weeks) and otherwise only that you can hardly say much more than "42" without a soil survey, such that it could "pay off".

My question now is: How do I proceed with the building planning (assuming the plot works out)?
[...] Do you look for a general contractor or architect before you look for a prefab house builder or a house from a specific provider? Or am I mixing terms here vaguely?

That you first look for who already has the (almost) suitable model in their catalog before an "individual planning for its own sake," I had already "praised." You would find such a model among the so-called prefab house manufacturers. General contractors in the masonry house sector unfortunately rarely still have catalog models (except the nationwide big names). With my advice, one can even end up with the architect if one wants to take such a type house / catalog model. "Necessary architectural services" aka "copying your floor plan (self-designed or pilfered from the internet) for the building application" you would also get from the general contractors / prefab house manufacturers. You can google externally my "House Building Roadmap, also for you: the HOAI phase model!" and the way to my personal advice in the same place; "my" Susanne certainly finances also in "40 km cheaper than Munich" ;-)

Briefly* summarized "how to proceed with the building planning" from my roadmap: You do "Module A" (basic evaluation and preliminary design) with a freelance architect and then a "rest period" (= phase of saying goodbye to the alternative designs not pursued) during which you ask a maximum of together a handful of wood and stone builders with the preliminary design, roughly where the price journey might go. After that, you either do with the same or a different architect either only the design and then hand over to a general contractor (the preferred way if continuing with wood construction) or the entire "Module B" (the preferred way if continuing with masonry construction). Note: a tender is not an alternative to awarding to a general contractor but is generally recommended!

*) here assumed for detached houses

So, sharing the property is an option, but rather in the more distant future, when we ourselves no longer need a larger garden. Next to the property is a green strip that the city wants to turn into a road at some point. Then there would also be a third building plot on the road. But that is all just talk for the future at the moment. For now, we are focusing on the 2 single-family houses.

Thank you very much for the reading tip. So you definitely recommend doing the preliminary analysis and conceptual design with an architect, even if we find something in the model home park that does not require too many changes?

And two more questions:

[*]Why are there hardly any houses with ground floor + first floor + attic in catalogs? Almost always only ground floor + attic.
[*]Is there an overview of which companies here belong to the big firms, the well-known budget providers, etc.? I skimmed through the forum with the providers but didn't find it easy to figure that out. Google only helps to a limited extent. Who is economy, standard, or premium? Who is known for problems, who for good experiences? Or is it really the case that this can’t be said in general, and everyone has to look for themselves? ("That depends" is always the favorite answer you get :))

Why are there hardly any houses with ground floor + first floor + attic in catalogs? Almost always only ground floor + attic.

It is probably because most development plans hardly allow that. Often 1.5 stories are desired. And even where two full floors are permitted, the eaves or ridge height usually spoils the plan if you want to use the attic as a full living space. Regarding the providers... do you know the Fertighausexperten? They have a rough classification per provider.