Multi-generational house with underground garage for 3 families

However, not everyone needs their own underground parking access, so an approximately 8.9 meter long and 4 meter wide driveway (40 sqm) is sufficient.
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Maybe I am interpreting something wrong, but this is a middle garage (>100m2 usable area, for 6 parking spaces and traffic area) and the following applies: (1) Ramps of medium and large garages must not exceed 15% [...] (2) Between public traffic area and a ramp with more than 10% incline there must be a less steep surface with less than 5% incline and at least 3 m in length. With a height difference of 3 m, this results in (2x3 m at 5% = 1.2 m and 12 m at 15% = 1.8 m) 18 m underground parking access. Ok, my 25 m was exaggerated; I had calculated using 10% max. gradient back then.

Maybe I am interpreting this wrong, but this is a mid-sized garage (>100m2 usable area, for 6 parking spaces and traffic area) and the following applies:
(1) Ramps of medium and large garages must not exceed 15% [...]
(2) Between the public traffic area and a ramp with more than 10% inclination, there must be a less steep surface with less than 5% slope and at least 3 m in length.

With a height difference of 3m, this results in (2x3m at 5% = 1.2m and 12m at 15% = 1.8m) 18m underground garage driveway.
Okay. My 25m was exaggerated, I had calculated at the time with 10% max slope.

You are probably right, if that also includes private garages. And then... yes, then... the idea of the underground garage on these 1000 sqm plus the wishes is simply an idea that should not be realized at all.
Then only the semi-detached house with a staggered floor remains, although my humble opinion is: an accessible granny flat has nothing to do with an apartment with a lift. What expectations are linked to the apartment?
It is only 1000 sqm, but these sqm must be evaluated differently in Munich than here in the flat countryside.

and . I really find your expert contributions very good and important. Exactly such comments are very interesting to me.

What expectations are tied to the apartment?

The expectations for the granny flat must be clarified in the short term. The expectations for the other two areas are actually clarified so far. Although the definition of clarified here is of course very broad!

We initially put the plan for the underground garage out there because in the case of two new buildings in the neighborhood (plot 1 approx. 890 sqm / plot 2 approx. 1100 sqm) an underground garage was each constructed...

It is only 1000 sqm, but these sqm are to be valued differently in Munich than here in the countryside

The constellation unfortunately has quite a high potential to end in private chaos. Please do not underestimate it; almost everyone has neighborhood disputes, and within the family, without clear - formal/structural separations, it will only get worse.

There will be so many situations during the construction where something does not work as planned and/or becomes more expensive. Then getting all 3 parties to agree again and again will not work. Do your parents understand modern Cat7 wiring and smart home technology, even if they don't need it, but the entire house system becomes significantly more expensive because of it? Do you understand your sister's increased need for security, even though it causes avoidable costs for higher security classes? Does your sister understand the durable and expensive clay roof tiles you would like to have? This all sounds a bit ridiculous and simplistic now, but in this situation, you will find yourselves in these conflicts at least 100 times, and no one can tell me that 3 parties are always of the same opinion, especially when it is always about mid five-figure amounts. Saying money doesn't matter is nonsense: everyone has a budget and sets priorities within this budget differently. These will always be very funny 2 vs 1 votes and the repeatedly overruled one will eventually be fed up. Bet on it!

thank you for the important notes.

From the past, I can only say that I have never had any problems within the family and I would be very surprised if it were to escalate. I myself am familiar with living in a multi-generation house and so far it has worked without problems (for a very long time...). Nevertheless, very important remarks that must be taken into account!!!

Is it possible to cheat on the underground garage by building a car elevator like that? That would basically be two floors plus a basement, so then you would "only" need three parking spaces and less traffic area in front. No idea how that looks with the costs, but compared to an underground garage and ramp maybe manageable? The question remains where to put it, since it would also have two stories. I also like the idea of the semi-detached house with a recessed granny flat on the ground floor.