Planning a multi-family house - optimizing costs

I would like to bring up another aspect: If Mom would like a spacious bungalow... how will she like living in a multi-family house? Even if you supposedly find "quiet singles/couples" as tenants – you can only judge people by appearances, and in the end, it's not peaceful at all because this clientele also occasionally or more often parties through restless nights. The more parties there are, the more likely this is.
Imagine you invest a lot in the project now (also financially) and in the end, she doesn’t like it and is utterly unhappy. Apparently, Mom has very specific ideas, and they surely go beyond the intended (really very generous for a single person) size.
To put it bluntly... she should build her bungalow – for you, it is probably also financially the cheapest if you directly support Mom financially, hopefully only many years down the road (you can already make provisions). Are you the only son?
If you have any influence at all, then make sure that the bungalow is well usable, also for a family of four (floor plan issue!), so that it can hopefully have good future reuse.
And I also find the advice from good – if such a person exists.

I would like to bring up another aspect: If Mom would like a spacious bungalow... how will she like living in an apartment building?

I had already mentioned that. But I would also turn it around. If Mom has such high demands or is "difficult," how will a tenant like living in a house where difficult landlords live? I know some places with frequent tenant changes when the landlord = owner = neighbor drives away his tenants and constantly complains about the costs of tenant turnover.

If you intervene at all, make sure that the bungalow is well usable, even for a family of four (topic floor plan!), so that it can hopefully have a good subsequent use in the distant future.

I only want to limit my "like" of your post in this respect, as I am known to assume that the market for retirement properties will already be able to function well as a closed system in a few years. Therefore, I increasingly see it as dispensable to design properties to be "universal" or even "family-focused" with regard to the age of the residents. I therefore do not consider a design of a bungalow optimized for a senior woman to be value-diminishing, as long as it is built roughly in a contemporary manner – radiator heating would be borderline freaky today. But, for example, a wall-mounted washbasin instead of one standing on a table does not yet represent significant minus points. So the house does not immediately become difficult to sell if it is specifically optimized as a ladies' bungalow or even a widow's little castle. But of course, "location, location, location" also applies here, and the market change I mentioned will certainly occur earlier in Stuttgart than in Geislingen.

How high would the price be for a nice hipped roof bungalow with you guys?

But I can’t get through with something like that with my mother...

May I ask how old she is?
I guess around 60 (You said the age is 39), then she is still in the middle of her life and her demands are understandable. However, even at this age she is still very competent, so she should actually be aware of the imprudence.

She would just build alone, with the mentioned consequences, or emotionally completely fall apart. I don’t want to impose either on her if I can prevent it.

My mother is now 75 (Yours would be that old in 15 years). And it really gets difficult, because rationality no longer works with older people who are at the crossroads between full competence and mental activity and the path of increasing illnesses and the resulting problems, which they do not want or cannot see. I hear only such stories of stubbornness at the moment.

Is it morally okay to deprive the mother of her legal capacity already now so that she is not allowed to realize her dream home?

Oh.. no!

she moves in, then pays me rent (which then goes through the tax lever).

As a first-degree relative, she only has to pay you half of the cold rent. The rest is a tax loss for you.

So now we’ve rather landed in the psychology forum ;-)

I think that’s actually the real problem here, the psychological one :(

Exemplary that you do not want to abandon your mother financially or mentally. However, the mom obviously and fortunately has her own ideas for her living situation, and in such a case you can actually only do wrong with all your alternative ideas.

You are building the multi-family house in which she herself is supposed to live. Is she capable of having "strange" people in the house, noise, sounds in the hallways when people come home and leave, parties/visitors, possibly pets, etc. etc. Later on, you will be to blame if she feels uncomfortable and would have preferred a bungalow after all.

I would try to implement the mother's wishes, but intervene to the extent that it might also be suitable for families later.

Sorry, this has nothing to do with your original question, but I see some parallels to my mom :D

We see all our mothers in your mother :D If it is meant to be, let her build her bungalow. Danwood or Scanhaus Marlow Marlow have nice floor plans. And maybe there is still some space in a corner for a small granny flat that she can rent out now, later it can be occupied by the caregiver. Well, and when the money is gone, one can rethink selling the house or something. Perhaps then she will be reasonable, won’t notice it so much anymore, or wants to go into care herself because she becomes more dependent.