Final floor plan planning - Rose-colored glasses

Bang.
I don’t think that wants to upset you, but certainly wants to warn you in time, and I think that’s right!
I don’t want to judge the structural engineering background of this expansion, but after reading the original thread, I have serious doubts about the sensibility of this project.
The question came up there as to why your parents don’t simply move into the upper floor, since it could be nicely designed for them in an appropriate way. Forcibly building something now because the parents apparently do not want to change their existing habits doesn’t seem like a good way to me.
You are submitting yourself and the possibilities for the family possibly to the understandable inflexibility or lack of willingness to change on the part of the parents (but I really don’t want to offend anyone).
Everything should stay as it is, preferably no change, but the positive things should come additionally (children, grandchildren in the house, etc.), maybe I am mistaken... then sorry.
I had an almost identical situation many years ago and ultimately decided against it... fortunately.
If someone lives in the master apartment here, then it is you with your large or even growing family, with working members, adolescents, increasing space requirements, and constant changes and new necessities in life.
Your parents may well have already finished their working lives, but their family planning is done; therefore, everything on their part will rather decrease than increase. Consequently, the house will be upside down.
My parents sold their house a few years later, we agreed on a kind of generational contract and lived wonderfully in a newly built house for almost 30 years. We in the ground floor with partial basement and my parents in a top apartment with a roof terrace in the attic, exactly tailored to their needs and much nicer than the old house.
This new house could later also be wonderfully resold as divided condominiums, then when your life changes over time.
If you write as an exclusion criterion that building land in Ruch is too expensive for a new building for you alone, why don’t you sell the parents’ house at a surely great current price and build exactly the house you want new, tailored optimally for both sides. Then the parents can live on the ground floor, attic, or separate annex, all of which can be done in size and demand.
But... older people often don’t want that because it’s too much change (which will come anyway).
I don’t want to pay the price (reduced living quality, unnecessary costs, etc.) of this lack of willingness with my “up-and-coming” family and submit to the, although understandable, but not really long-term thought-out wishes of the parent generation.
A plus/minus list for a sale/new build would quickly show you what the appropriate path for you is, because despite all the love for your parents, you are primarily responsible for the well-being of YOUR family and only at some point long afterward for the desire for minimal change on the part of your parents.
I hope you can take it as I intend, merely as a thought-provoking impulse to possibly find the courage to think completely anew!

I don't think @11ant wants to annoy you,

Right. In real life, he is really nice. By the way, also here.

I don't want to judge the structural engineering background of this extension

That is definitely possible even for laymen; for that, you mainly just need to imagine the shown floor plan "in color extracts": the yellow crossed-out part is the demolition, the gray is the sparse intersection between old and new, and everywhere the exterior walls protrude in red like the fingernails of supermarket cashiers, that's where the treehouse crushes the existing little house flat and turns it into a dark cave. In the original thread, it can be followed more easily, but even here it can become apparent that here probably Eulenspiegel is spurring on. The practically downward-open gallery staircase, attached like a fire escape, finishes off the marketability of the existing house. Not as a mercy shot, but rather as a sneaky neck shot, mind you. The poor parents would take a rope even during the shell construction if they realized what they signed because, on one hand, their dear little daughter wishes it so much, and on the other hand, they believed they could grow old in their familiar surroundings. But as I said, I firmly believe that the bureaucracy, which is otherwise so often criticized, will pull the emergency brake.

And then they watch and listen to the underwear spinning around?

I don't really find that so bad, it's right outside the door. Many condominiums have separate laundry rooms in the basement. Only in combination with an office somewhat impractical.

But probably noises will come through. So the customer won't just notice the washing, but also the family's toilet visits?

And then they watch and listen to the underwear spinning around?

I don't really find that so bad, it's right outside the door. Many condominiums have separate laundry rooms in the basement. Only in combination with an office somewhat impractical.

But probably noises will come through. So the customer won't just notice the washing, but also the family's toilet visits?

No, the washing machine and dryer would be completely concealed so that they are not visible. Customer traffic in this office is very rare. However, it could happen occasionally. You don’t have to do laundry there when a customer comes.

...and make it warm.

No, I also think that is not an option. And customers are supposed to be received in the office??? Next to the pile of dirty laundry or the running washing machine when the underwear passes by the washing machine window? Not serious!

Customer traffic is very rare - if there is ever one - the washing machine is installed so that it cannot be seen.

Your children are supposed to shower for years in a 2.89m2 bathroom? At 15 they will refuse.
Even a 1.34m2 guest toilet is very tight. Take the wall out there.
If you really get a spontaneous visit: before they’re upstairs, you quickly check the bathroom again and just throw any lying-around stuff into the kids' room.

Well, I would have been happy at 15 to have my own shower and my own mirror with my "makeup stuff". The small bathroom made me insecure too. But I was at a bathroom exhibitor yesterday and they said the size is completely sufficient and I shouldn’t worry about it. I also grew up with such a small toilet—a small quiet place—no, I’d rather have a small toilet than the one inside the kids’ bathroom.

Apart from the bath and washing machine solution, which I do not like, I agree with 11ant 100% (I just read the original thread). This will be a structural monster. Out of the question. I also hope for the official veto.

I can quite understand that the parents do not want to change their environment. It was the same with us. Originally, we wanted to renovate my parents' house - but no way! We were lucky to be able to build next door. If that hadn't worked out, we might have waited for a suitable building plot or for my mother to no longer live in the house someday. Whether she would then be in a nursing home (which I never wish for her) or deceased - whatever. But then we would have renovated the house for ourselves and probably lived with a temporary solution in the meantime (because the chance of an affordable building plot is rather low). But such a monster would never have occurred to us even in our wildest dreams.