Who signs the prefabricated house purchase contract?

Yes, now it is more understandable :)

Of course, fairness is also a consideration! You can't just benefit from a house; you should also share the liability, meaning contribute to the financing. Besides, banks like the security of two debtors – usually, you won't get good financing on your own, meaning you get financed less for one salary.
A [Werkvertrag] should be signed together for this reason, because then each person alone can bear decisions and be the contact person for the construction company. It can happen that one person is unreachable due to work reasons... or is sick, and if that person is then the sole buyer or client, problems could arise.

Hello,

in advance - we are not allowed to provide legal advice here, as this is exclusively reserved for advisory professions in Germany.

We, my partner, our son, and I (we are not married yet) want to build next year. My husband has mostly taken over the financing for the plot of land we bought half a year ago. I am included in the purchase contract of the plot. Do I or should I co-sign the purchase contract of our planned prefabricated house? If I am not included, then the house does not "belong" to me either, or does it look different anyway because we are (still) not married? Or should I better not sign to avoid being liable for the high loan debts in case of an emergency.

This is not as easy to answer as you obviously expect.

The plot belongs to you in equal shares if you have not made any other agreement (for example, according to the monetary share) in the associated notarial contract. The prefabricated house will be built on your joint plot - regardless of whether you sign the construction contract or not; therefore, it is connected to the plot and can no longer be separated. This can lead to considerable problems in the worst case if no binding agreements are made in advance.

Whether your husband can manage the financing alone or not is, in my opinion, initially of secondary importance. You should - this is how I see it, as no one can look into the future - draw up and sign a notarial contract for the case of emergency at a time when you get along well. By the way, you could also directly transfer "your" share to your joint son ....

And finally - if the financing later becomes unmanageable for whatever reason, it does not matter whether it belongs to your partner alone or to both of you. AS LONG as you get along well, you will both pay from one pot :-) Otherwise, it is always good to be able to fall back on legally binding agreements.

Rhenish greetings

Can you make any use of our answers or could we have spared ourselves the words??