Renovation of an apartment in the parental home - loan, without being the owner?

we are planning......

My husband will of course contribute to the furniture....

Hm.... you are married and plan together but only one pays or wants to own it alone?? You are remodeling your parents' apartment..... your husband pays "something" into it..??
That sounds extremely shaky, fingers crossed that it all works out.
Why don’t you split it into condominiums and "buy" one from your parents, then it belongs to you, your husband, and you. A tax advisor/notary can give you feasible options.

I’m surprised that no one here warns against putting money into a house if you are not listed in the land register.

yep
You are only considering this from a financial perspective. Such models can, however, cause a variety of interpersonal problems, e.g. when life changes…. for whoever.
You should become the owners of the apartment and then really jointly plan/finance your apartment with your husband. You can then legally regulate the ownership shares/conditions privately in the marriage through a notary.
If the parents don’t want to give you the apartment, which is their right, I wouldn’t do it. I don’t understand your husband, how/why he would go along with this. In his place, I would rather have my own apartment. You can live well like that too.

If the apartment does not belong to you, you should (at least formally) pay rent.

Your parents should take out the loan and the costs they incur could be reflected in the rent. That way it would be clear and straightforward!

Best regards
GD

So we did something similar eight years ago. However, our house was divided into two condominiums according to [WEG]. My parents transferred the attic to me in a shell condition, and I in turn transferred half of it to my husband, so we both appear in the land register with 50% each. We then financed the conversion together. I just thought it was fair, my husband also earns the majority of the family income since I work part-time because of the children. At the notary appointment, we were asked if a right of first refusal should be registered for anyone just in case. I would never invest money in a property without being an owner.

If the apartment does not belong to you, you should (at least formally) pay rent.
Your parents should take out the loan and the costs incurred for it could be reflected in the rent. That way it would be clear and clean!

The tax office will assume rent to be paid anyway.

However, in our case the house was divided into two condominiums according to WEG.

yep - a clear regulation!

Do you have siblings or are you the sole heir?

Would divide the house into residential units.

Why get married if, at 120k, you're having such a hassle with your husband??? It's supposed to be until death do you part, but in Germany it's often about the first euro. Probably separate accounts on top of that... if my partner came with something like that I would have zero interest in all the stress and would question my relationship.

Divide the house into apartments and together buy 50/50 each apartment from your parents and take out a joint loan. A we often connects more than €5.50 extra in the other's account....