Additional costs for residential units

I didn’t expect "volume discounts." I actually wanted to approach it the other way around: What can I do myself in terms of planning to design multiple residential units as cost-efficiently as possible?

That’s not the other way around, but maybe what you want would be better called "synergy effects."

For example, stacking bathrooms and kitchens on all three floors. Does that help? Or is that completely irrelevant in terms of construction costs?

Concentrating the lines is already sensible in a two-and-a-half-story single-row building, but even the twenty-first residential unit doesn’t pay for itself yet ;-)
Better build a nice house primarily for yourself instead of already feeling like a big property developer just because of a possible second granny flat :)

Is that so? I have currently planned it with a gradient of 30-35 degrees optionally. This still results in considerable living space when expanded, which then suffices for an apartment for the child (especially in the first approximately 5-10 years during/after training/studies).

You are resistant to advice: there will be no second escape route under a tent roof. But a self-contained residential unit needs that.
Many questions do not change the facts.

You are resistant to advice

Or you are unfriendly ;) Sorry, this really is a very informative forum if you get the right people. But what I have partly (so far especially in other threads, now you are also starting here) read in terms of insinuations, generalizations, and sometimes insulting judgments stands in stark contrast to what this forum (in my opinion) should be about.
To the point: Why should I be resistant to advice? If an escape route is feasible there according to the architect, then what is the problem? There are special roof windows that are suitable as escape routes. Why shouldn't you be able to install them in a tent roof with a 30-35 degree pitch?
Advise me, then I won’t be resistant to it. But that requires a bit of information so that your (of course nowhere near as all-knowing as you) counterpart can understand you.

Many questions do not change the facts.

You always start with many questions when you do something for the first time. The real question is rather how these are received or in what environment you can ask them. There are supportive environments... or not. Then answers to questions sound condescending, hardly help, or even fail to appear.
You rightly state that many questions do not change the facts, but that is not the point. The point is not to change the facts, but first to understand them and, if necessary, to look for viable alternatives. If my plans don’t hold, I’m the last one to want to override physical laws. But if they do hold (or problems are solvable), I can also implement them.
For example, my question about the cold roof has so far gone unanswered. Or the one about whether insulation of a tent roof is particularly expensive. Why should I then be resistant to advice if I haven’t yet heard any counterarguments against insulating and finishing a tent roof?

A stack concentration is also already sensible in a two-and-a-half-story single-family house, but even the twenty-first residential unit does not pay for itself yet ;-)

I did not expect such extreme savings either. But I basically have no problem planning that way. If it slightly reduces the costs when laying the pipes (even if only 1-2%), that would already be something.
Is there a thumb rule for what it costs to lay the pipes/connections for kitchen and bathroom to the upper floor if you do it one above the other and if you do it across the house?

Is there a thumb value,

No. For multi-family houses with 0.25 dozen residential units, there is no student of housing economics worldwide who would want to make that the topic of their master's thesis.

Or you are unfriendly

I am not unfriendly, but have given you factual answers.

There are special roof windows that are suitable as escape routes.

We have already discussed this topic! See:

Even though I don't know what the house looks like, I assume that your apartment was not in the attic, and if it was, then with dormers or at least one window that served as a second escape route. Many regular windows are second escape routes; a roof window is not one.

But what I have partly (previously especially in other threads, now you are also starting here) read in terms of insinuations, generalizations, and partly insulting judgments stands in stark contrast to what this forum (in my opinion) should be for.

Enough now! Get your information from this thread and learn to understand. But not just because you don’t get what you want to read.

There are special skylights that are suitable as escape routes.

Yes, Yvonne went a bit too far there. But I didn’t want to argue about it last night, especially since exceptions like these are too complex a topic and would lead too far off topic here.