BV single-family house with full basement

The calculation lacks any allowance for patterning buffer, outdoor facilities, furniture/kitchen, carport or garage

I have always found it puzzling how one can assume to estimate the costs so precisely at such an early stage of planning.

I addressed carport/garage in the original post. Furniture is available, we live in a very large apartment. The kitchen is budgeted at 10k (clearly, not high end). I agree with you on the patterning buffer, but it should be limited with the company. That leaves the outdoor facilities including the garden. I should maybe add that our household income should improve significantly in the future, so I don’t see any risk that we will have to live for years without a terrace, garden, fences, etc.

How is it actually with this potential quasi-property developer situation: Is it ensured that in the end real estate transfer tax does not have to be paid on BOTH the land AND the house, if the BU is significantly involved in the land acquisition?

I also addressed this aspect immediately myself. It is perfectly clear to me that a strict separation must take place here.

I would stay away from it and honestly advise you against it. Better do the door-to-door visits and other acts of desperation yourself to find a plot separate from the provider. Or sign a proper developer contract right away, where you know the plot beforehand.

Do you know satisfied previous customers of this provider? Specifically those you know yourself or found by asking around, not those named by the provider?

What I liked best during our search were the developers who named reference projects but said, "please please do not ring there, we send so many people that the residents are incredibly annoyed." I once spoke to a former customer when he was standing in the garden; he was very talkative about how the construction went. Afterwards, I definitely did not want to build with that company anymore.

Asked the other way around... what sense does a contract make for the provider if you can cancel it at any time without restrictions and damage (money)? What commitment does the provider expect from it?

A free right of withdrawal would be agreed upon, provided no suitable plot of land is found. The advantage when concluding the contract is that the prefabricated house company helps with the mediation of suitable plots of land. Of course, they wouldn’t do that if you don’t conclude a contract with them... through the right of withdrawal I don’t see any disadvantages for us or have I overlooked something?

I suspected it...

I also immediately addressed this aspect myself. It’s completely clear to me that a strict separation must take place here.

But we are not dealing with wishful thinking here. If you sign this path, then it is like that.

3. I urgently need a separate office, as I mostly work from home (planned in the basement, see below).

That would make me completely depressed, working full-time in a dark basement - unless a large sloping window front is planned.

I would rather move the guest room to the basement then (well, effectively sooner or later you will anyway convert the above-ground guest room into a home office).