Planning a multi-family house - optimizing costs

Especially with a long-term investment horizon, the market offers many good entry opportunities ;-)

A sensible investment!

If she spends the rest. In your scenario, she spends the rest on building the house. Do you think the risk is lower in a multi-family house after 10-15 years? There the risk is actually several times higher, so that something must be done beforehand (and regularly). With every tenant change. Much more technology. Problem tenants. Wear and tear of the house by strangers. Etc.
In my opinion, you are imposing a higher risk on her (with significantly lower living quality).

That is complete nonsense. The WKR poses absolutely no problem for older people.

For YOU, not for the mother ;-)

If my mother builds her bungalow, she doesn’t have enough equity left to, for example, generate enough income with a "safe" dividend stock so that it covers living expenses and incidental costs together with her pension. She insists on the bungalow and the size of the apartment. Unfortunately, that costs money. Therefore, as said, she would probably have to sell at times when she fully experiences it.
Renting is out of the question for her.

What would you consider a sensible investment? My family has already seriously burned their fingers several times even with "guaranteed safe" investments. And as I said, I assume that we will settle rather statically like Japan in terms of, for example, the DAX.

Regarding WKR and older people, I received different information from my house bank and the prefabricated bungalow manufacturers where we originally were.

I don’t really see the advantage for me. I hope that she manages the money sensibly for herself and preserves or increases it if possible, i.e., lives from a dividend from it, however that looks. Whether that is the case with a safe investment or in a house is totally irrelevant to me.
For me, it would be more of an advantage that I can then implement my plans with apartments for my family more easily, as I am not pumping so much credit into an investment that binds me for a long time (which is currently only running on me in the current plan), but am more flexible and can thus take advantage of good opportunities more easily.

As said, I am open and thankful for concrete suggestions. ;-)

I summarize:
The mother’s money is enough for a bungalow, but not for care.
Instead of the bungalow, the residential unit in the planned building is paid for. It costs just as much, leaving just as little leftover.

And now there are an additional four residential units, which after 10 years pay for themselves and generate enough to pay care costs permanently without worries? That can’t work...

That’s where my original question at the start of the thread comes from. (how much can I reduce the total price per sqm) With the previous offers for bungalows, etc., it was significantly above the previously targeted 2900 € per sqm for a large building. Her living area would therefore have been considerably more expensive. With the money she saves compared to the bungalow, plus part of the rest that she would have otherwise spent, she could pay for part of the two apartments planned for her. For the rest, I go in with my own capital and a loan.

She insists on the bungalow or the apartment size.

That would be my starting point. A clear message to mother: "The money is not enough. What now? Will you stay in your current apartment or what concessions are you willing to make? Well-planned 95sqm without a basement should be enough for you, right? Ok, then let's start building!" Anything else is not serious.

That would be my starting point. Clear message to mother: "The money is not enough. What now? Are you staying in your current apartment or what concessions are you willing to make? Well-planned 95 sqm without a basement is enough for you, right? Ok, then let's get started with the construction!" Everything else is not serious.

I totally agree with you. She insists on at least 110 - 120 sqm of living space plus a utility room / basement or raised roof for storage in the bungalow. I've been trying to talk her out of needing that much all by herself for, honestly, 2 years. The size we have as three people in an apartment, and it's more than enough. We would even downsize once my daughter moves out. But with my mother, I can't get through with such arguments...

I completely agree with you.
She insists on at least 110 - 120 sqm of living space plus a utility room / basement or raised roof for storage in the bungalow. I've been trying to talk her out of needing that much on her own for, no joke, 2 years.
We three have that size in an apartment and it's more than enough. We would even downsize if my daughter moves out. But I can't get through to my mother on something like that...

Therefore, "unfortunately," this is the starting point I have to build on, and from this issue the whole plan results.

I have been trying to convince her for honestly 2 years that she doesn’t need that much on her own.

A lady of the world doesn’t just need a shoe room, after all, the clothes still need to go somewhere. And hats, of course.