Building as a single, marriage, and prenuptial agreement

Well, the cost of living is about half of the additional housing costs. That's why I actually see it a bit differently. She basically lives rent-free and everyone pays you half of the additional housing costs. I think that's pretty fair. But as I said, that's not what it's about.

Only from the increase in value during the marriage.

I can think of dozens of things that can be arranged beforehand. That saves you tens of thousands of € in lawyer fees and years of stress after the separation. But this goes too far here.

Tens of thousands at the lawyer. Aha...

We agree as long as there is no child yet and both work full time. [...] I am now rather considering negative points that could come up for her. What happens if I can no longer pay the installment or what other points make a prenuptial agreement reasonable.

At first glance, I already see one question that makes a prenuptial agreement sensible: to clarify the conditions under which you still agree if there is a child and the family work might be divided in such a way that both no longer participate equally in gainful employment. The mere fact that you will be jointly assessed fiscally in the future and can influence your tax classes shifts many things (or opens up unforeseen areas for disagreements) – from my (fortunately not own) experience, enough to imagine disagreements that suffice as grounds for divorce.

I have also considered transferring 25% of the house to her if we split the last 50% of the payments.

Even if minus times minus equals plus, nonsense twice does not become sense.

but I think she doesn't want that.

From a future wife one should not believe anything about what she might like or dislike, but ask her.

I pay the installments + additional costs, my soon-to-be wife pays for the groceries.

From the wedding onwards, both of you legally pay for both, even if the fact that you technically pay this and she pays that does not lead to a settlement. I recommend couples to conduct a virtual divorce annually at the end of the year and to make all compensations in bookkeeping, then in the case of divorce only the current partial year has to be settled. Put simply: being "two mates in agreement" as a "double income, no kids" couple is no art. A leading reason for divorce nowadays is the relapse (especially of the man) into old role models after the girlfriend has been married. If you now think, no no, you are not such a person:

, granted.

... this inconspicuous addition makes my feminist alarm bells ring.

Tens of thousands at the lawyer. Aha...

Discuss for 3.5 years about separation maintenance, post-marital maintenance, child support 3 times as well as equalization of accrued gains in let's say not insignificant amounts and the division of household goods.
Notaries and lawyers are like money-printing machines.

There are so many good reasons for a prenuptial agreement. A good reason is to protect the partner from insolvency. Contract comes from "to agree" – so you write down what both find fair and reasonable. It's actually quite simple. We have a simple prenuptial agreement that separates assets, protects against the assumption of losses, and bypasses the increase in value but keeps the pension claims intact. Since we had a lifestyle concept that amounted to a single-income household, I wouldn’t have found the exclusion of pension claims fair. My wife probably would have signed everything that day. The notary thought the latter was "not so smart" of me. The contract has now been in place for 26 years and we still get along.

Sometimes I think we focus way too much on things going wrong instead of on things working. If it's true that energy follows attention, then one could see a connection between the idea of protection and separations.

Discussing for 3.5 years about separation maintenance, post-marital maintenance, 3x child support as well as equalization of accrued gains in, let's say, not insignificant amounts, as well as the division of household items. Notaries and lawyers are like money printing machines.

I fully agree with you on the last sentence! On the other points, conditionally. Because whoever sinks tens of thousands into lawyers certainly has MUCH more than 95% of the members here in the forum.