Paying "rent" to the partner... how?

Do you then reduce the payment by a few cents every month because it is being repaid? Otherwise, I also find that a fair model.
And you don’t have to tell the mother-in-law, who does seem to fit the cliché, too many details.

In my opinion, this should not relate to the actual interest on the loan but to the long-term secure (cf. real estate) capital interest rate of the house’s equivalent value. So, for example, the fixed deposit rate for 10 years. Especially if a (large) part of the property is financed through existing equity (e.g., also family), I see this as the fair solution.

You are apparently mainly thinking of risks during the construction phase, which is a shortened view. Although the construction phase can also go wrong despite safeguards, as is repeatedly read in this forum.

He bears the risk for defects and wear.

He bears the concentration risk, becomes illiquid, must endure value fluctuations.
His creditworthiness is affected.

Who pays increased property taxes, garbage fees, or five-figure construction surcharges for the new road?

It is not only about money but also time and responsibility.

These issues, and the list is not complete, are often ignored.
This leads to the perception that owner-occupied properties are always a great retirement provision and renting is wasted money. That is simply not true.

I agree with you on the downstream risks! But these then belong to the maintenance of the property and not to the pure purchase or construction.

If all of this is such a no brainer, why don’t all people do it?

Apart from the fact that building is often mega stressful, also for the relationship..

You can become unemployed, who then pays the installment?
And a house can also lose value, that must not be forgotten just because everyone here has been able to benefit from rising property prices in recent years..

My grandparents have an apartment in the Harz... it used to be worth a lot... now no old s** wants to go there and renting it out is almost impossible...

Not everyone feels like taking on all this risk, etc., but with that opinion you’re also a bit in the wrong place on a house building forum...

He bears the risk for defects and wear. He bears the clumping risk, becomes immobile, has to endure value fluctuations. His creditworthiness is affected. Who pays increased property taxes, garbage fees, or five-figure construction cost subsidies for the new road? It's not just about money but also time and responsibility.

He bears the risk, but it is his decision to bear this risk alone. If he would have bought the same property anyway, with or without me, then if I would not move in, he would bear this risk alone just the same. So, essentially, any payments I make are pure profit for him, as I do not increase the risk. I only increase the ancillary costs for electricity, water, and garbage, nothing more.

After careful probing, unfortunately, he now argues in the direction: You would have to pay rent anyway. It remains exciting.

If this is all such a no-brainer, why don’t all people do it then?

Apart from the fact that building is often mega stressful, also for the relationship...

You can become unemployed, who then pays the installment? And such a house can also lose value, you must not forget that just because everyone here has been able to benefit from rising real estate prices in recent years...

My grandparents have an apartment in the Harz... it used to be worth a lot... now no old s** wants to go there anymore and renting it out is almost impossible...

Not everyone wants to take on all this risk, etc., but with that opinion you’re somewhat at the wrong address in a house-building forum...

I have already written that good and thus smart planning is necessary! That not everyone can or wants to afford a single-family house, you know as well.

The topic of dismissal and loss of income is a matter of its own. One should really consider how high the probability is that one can pay the installment in the next 10 years. If I know before buying a house that in 12 months I’ll be distributing brochures on a 400€ basis because my employer currently has a lull in orders or I feel like getting some fresh air, then I don’t build. If the relationship is rocky or I’m currently cheating, then I don’t build. But yes, those cases exist!

It’s also clear that our houses, if you don’t live in Hamburg, Munich, Berlin, Stuttgart, or Frankfurt, won’t be worth what we paid for them in 20 years, right? If after 15 years a sewage treatment plant is built right next door, then you just had bad luck.

That’s all an old story! I’ve been here long enough for that. It has been very often proven here that if you rush into something like this without thinking or under time pressure, then you don’t always get the optimum result, then it’s no no-brainer, true. But then it’s your own fault.

But I don’t want to be a smart aleck now. There are plenty of factors you can think through to make building and maintaining a house easier for yourself.

He bears the risk, but it is his decision to bear this risk alone. If he would have bought the same property anyway, whether with or without me, I would not move in, he would bear this risk alone just the same. So actually, any payments on my part are pure profit for him, because I do not increase the risk. I only increase the additional costs for electricity, water, and garbage, nothing more. After cautious probing, however, he now unfortunately argues in the direction: You would have to pay rent anyway. Remains exciting.

Ask him what he thinks about you finding your own place and moving in there. He really wants the house. And since you don’t want it, I find the split with 50% interest and 50% additional costs plus 100€ for depreciation fair... Without him you would have to pay rent, yes, but he without you would have to pay everything completely.