Realistic cost estimate: Single-family house with unfavorable development location

We have budgeted €80,000 for the development and the assumed €40,000 is already our share.

But that doesn't sound fair to me. The joint development goes up to the building partner, the section after that is solely your responsibility. So it can never be fifty-fifty. The dispute is already inevitable.

What else can be said. You will not make it with the 400,000 EUR. But just do it, it has really been said often enough.

This is the offer from our construction company, not in basic equipment, but after recording our wishes and with buffers for some things we don't even want (satellite dish, smart home, etc.). We have to work with this somehow, right? Yes, that's how it's intended. We have made other concessions for it and it is undisputedly agreed that we will do it that way.

Oh okay, well, hopefully our needs analysis from the beginning was already there for that.

I don't recommend anything that has already become obsolete or even ridiculous. Visualization often presses a turbo button.

Yes, that is the intention. We have made other concessions for it and it is undisputedly agreed that we do it this way.

Then you are lucky. Good for you. :)
I would definitely put that in writing. Even if it is the favorite brother or the nice aunt. Once construction has started and the money melts away like butter in the sun, it’s good if nothing can be changed anymore.

This is the offer from our construction company, not as a basic package, but after taking into account our wishes and with buffers for some things we don't even want (satellite dish, smart home, etc.). We have to somehow work with that, right?

And I say, plan in another 10%, better 15% reserve. If you don't believe me, fine...

Our furniture is coming with us and we don’t need anything big new for now.

How many rooms and how many square meters do you have now? Probably significantly less than in the house. It just adds up, simply because you have more rooms and space.