Prefabricated house including land planned - financing

Yes, thanks for the tip. You’re probably right, it could be that he wanted to badmouth the other provider, I can’t really judge that. But a fixed price legally means that nothing extra can be added on (except of course if you yourself want to change something additionally), right?

You often see houses where the text says "Sale of the property is conducted through one of our partners".

It hardly gets any more miles away from hidden builder model stench ;-)

What is a Bauherrenmodell?...but still...with an offer like mine where it says fixed price, it shouldn't be that easy for additional costs to come up, right? So planning, ancillary construction costs, etc. are indicated as included. Or are there exceptions where these can increase or the builder can still charge more?

Wasn’t that this ad?
I don’t read anything about a fixed price.
Is it a developer model, is there a purchase price, i.e. a so-called fixed price? You buy, the developer is the builder.
In your case, the package deal: You sign a contract for work with a construction company at a fixed price and become the builder on a plot of land that you buy yourself. You become the builder and have full responsibility for the plot. There is no fixed price because a plot of land can result in "risk work."

P.s. I remember that you posted the link to the offer. Since links are prohibited here, your thread was consequently deleted. Be glad that you were not deleted here, as the admin usually performs a clean slate when rules are violated.

What is a Bauherrenmodell?

That would be the tied deal from the fairy tale, IF it weren’t for the little word "if" ;-)

with an offer like mine that says fixed price, you can’t just have extra costs added, right?

Yes, "right" is correct ;-) If you want a fixed final price guaranteed without any ifs or buts, "paid as agreed," you basically need a time machine: because this would have to be a real classic property developer offer, which today only exists as an Easter egg jackpot on a full moon, basically like a Blue Mauritius with gold trim and a cream topping. The real classic property developer offer (house is actually finished at signing) practically no longer occurs in the wild because—keyword "Basel"—property development is no longer a "business" in terms of "profit" due to liquidity and yield reasons. When this began to go out of fashion, the chancellor was still called "Birne" (pear). It’s not completely extinct yet, but even with "connections," you have to look for it with a magnifying glass. So stop dreaming, and set aside money for inflation and similar risks in your war chest. With counted-out money, you can no longer build a house today; in a really serious worst case, you can’t downgrade tiles and sockets enough to even then still guarantee it will be enough.