Final floor plan planning - Rose-colored glasses

So I would never put a washing machine and dryer in the office. Especially the dryer is quite loud and generates a lot of dust. And with four people, both devices run regularly and simultaneously with office use.

Western Austria

Vorarlberg? Bregenz area?
Why this Kitzbühel chalet nouveau riche style then?
I find the architectural style in the Vorarlberg area very pretty and suitable for an extension. Stay regional stylistically!

Overall, I have to agree with my predecessors logically: this will be a monstrous building where the upper part does not fit the lower one. The staircase to the upper floor will probably be removed downstairs. This will result in an already oversized hallway gaining even more useless and dark space. In the long run, this means: a house with two such suboptimally designed apartments will be hard to sell.

What happens when the parents-in-law have passed away? Will the ground floor be rented out? Huge area, but only suitable for a couple because another room is missing. Instead, a ballroom as a hallway. All these slanted walls that take up space and make it impossible to furnish the whole (except with custom-made built-in furniture) sensibly.

If it’s not a complete new build already, I would at least consider larger renovation measures on the ground floor.

And then comes: the in-laws do not want any changes.

I know this, we had the same here. Instead of a new build, we initially planned a renovation of my parents’ house; my mother was supposed to get a nice, age-appropriate granny flat. She’s over 80 and lives in our family home with a ground area of 140 sqm. She refuses a cleaning lady, but this living area is hardly manageable for her anymore. But no, she didn’t want any changes either. She basically built the house with my father on her own. Every stone holds memories and half a life.
No, it makes no sense to keep an over-80-year-old in a house designed for a family of four. But is it sensible to take away all memories from an old person? My mother would have been unhappy even in the nicest, most age-appropriate granny flat, always thinking about the old house that was hers and is now the granny flat, which it no longer is.
On the other hand, we would have had to make many compromises in the renovation. The biggest: we actually didn’t want a granny flat but then would have had one. What would have happened to it after my mother’s death? We didn’t want strangers moving into the house.

From this point of view, I can very well understand your current situation. Even if it would be sensible, you can’t just take away the living environment of old people, even if logically and well would be different. Life has more components than "logical" and "sensible." And these other components are indeed important and must be considered.

Nevertheless, I would not turn this already (in my opinion) completely lousy floor plan (which was once stylish but is now so impractical and wastes space unnecessarily) into a real monstrous thing with this addition.
What do you get afterwards? A ground floor apartment that is basically unusable after the in-laws’ passing, as it’s far too large for typical 2-room apartment seekers and not usable for a family with only one child because that single child’s room is missing (you just can’t put such a child in the hallway). Meanwhile, you sit upstairs in an also not optimally designed apartment.
It’s possible that a second fundamental renovation will then follow and the living areas will be remodeled for the then-grown children, and downstairs for you or vice versa or who knows.
More renovations, more tinkering with a solution that was not already optimal in the starting situation.
That would all be too annoying for me.

We were lucky that the plot was big enough for another house; apparently, that’s not the case for you. Too bad.
But until we had this on our radar, our motto was: we save until my mother can no longer live in the house. Because she has died, because she is in a nursing home, or whatever (she married a young man and it would have been possible to build again with him *g*), and then we build or tear down the house calmly and exactly according to our ideas. Exactly as we want. Without half-baked compromises.
And I really recommend this solution to you in your current situation.
I don’t want to spend a higher six-figure sum on a solution that is just “it works” or “okay.” That’s so much money, I want exactly what I imagine. For a “that’s alright” solution, that’s too much money.
Find a good interim solution, save money for later, and then renovate the house accordingly or tear down this built-up thing that really only consists of a large, unusable hallway, and then build something for your then existing situation that fits you 100%.

Just my 2 cents...

Vorarlberg? Bregenzer Raum?
Why then this Kitzbühel chalet nouveau riche style?
I find the architectural style in the Vorarlberg area very pretty and fitting for an extension. Stay stylistically regional!

Overall, I have to agree entirely with my predecessors in terms of logic: this will be a monstrous building, where the top does not fit the bottom. The staircase to the upper floor will probably be removed downstairs. This will result in the already oversized hallway gaining even more useless and dark space. This means in the long run: a house with two poorly laid out apartments will be hard to sell.

What if the parents-in-law eventually pass away? Will the ground floor be rented out? Huge area but only suitable for a couple because another room is missing. Instead, there’s a ballroom as a hallway. All these slanted walls that take up space and make it impossible to furnish everything reasonably (except with custom-made built-in furniture).

If not a complete new build, I would at least consider major renovations on the ground floor.

And then comes the: the parents-in-law don’t want any change.

I know this, we had the same here. Instead of a new build, we first planned a conversion of my parents’ house; my mother was supposed to get a nice, age-appropriate granny flat. She is over 80 and lives in our family house with a footprint of 140 sqm. She refuses a cleaning lady, but this living area is hardly manageable for her anymore. But no, she didn’t want change either. She and my father basically built the house themselves. Every stone holds memories and half a lifetime.
No, it makes no sense to keep someone over 80 in a house designed for a family of four. But is it sensible to take away all memories from an elderly person? My mother would have been unhappy even in the most beautiful, age-appropriate granny flat, always thinking of the old house that was hers and now the granny flat, which it no longer is.
On the other hand, we would have had to make many compromises in the renovation. The biggest: we actually did not want a granny flat, but would have ended up with one. What would have happened to it after my mother’s death? We wouldn’t have wanted strangers in the house.

Therefore, I can very well understand your current situation. Even if it would make sense, you can’t simply take away the living environment from the elderly because “logical” and “practical” would be different. Life has more components than “logical” and “sensible.” And these other components are certainly important and must be considered.

Nevertheless, I would not make an already (in my opinion) totally bad floor plan (which used to be nice but is so impractical and wastes unnecessary space) into a real monstrous thing through this extension.
What will you have then? A ground floor apartment that will be basically unusable after the parents-in-law pass away, because it is way too large for the usual seekers of 2-room apartments and not usable for a family with only one child because there is no child’s room (one child can’t just be put in the hallway, after all). Meanwhile, you sit upstairs in a not optimally laid-out apartment either.
Maybe then the second fundamental renovation happens, and the living areas are rebuilt for the now grown children and downstairs for you, or vice versa, or who knows.
More renovation, more tinkering with an already not optimal solution from the start.
That would all be too annoying for me.

We were lucky that the plot was big enough for another house, which doesn’t seem to be the case for you. Too bad.
But before we had that on our radar, our motto was: we save until my mother simply can no longer live in the house. Because she dies, because she is in a nursing home, or for whatever reason (she married a young man and building again would also be possible *g*) and then we calmly renovate the house exactly according to our ideas or demolish it and build new. Exactly as we want. Without shady compromises.
And I really strongly recommend this solution to you given the current situation.
I don’t want to spend a higher six-figure amount on a solution that is just “okay” or “good enough.” That is so much money, I want exactly what I imagine. For a “good enough” solution, it is too much money.
Find a good interim solution, save money for later, and then renovate the house accordingly or demolish this built mess, which basically only consists of a large, unusable hallway, and then build something 100% fitting to your situation at that time.

Just my 2 cents...

Yep, because neither the parents should make uncomfortable compromises for themselves nor should the young people, because of the possibly understandable immobility of the parents, tie their own lives to it and end up with a “lazy” compromise at such a high cost. Usually, one side remains dissatisfied and eventually that can backfire.
So maybe you can’t live together directly, or you should look for solutions where neither side is so constrained.
I’m curious…

Just my 2 cents...

Oh, penny stocks often don't have bad price performance at all

The situation in my parents' house was similar. An old farmhouse, built in 1804. My parents assured my brother that he could do whatever he wanted with the renovation.

However, when it came down to the nitty-gritty, that was no longer the case: this was not allowed to be done, that was not allowed to be done. All suggestions were nipped in the bud.

Ultimately (after 3!!! years of planning phase): partial demolition of the rear half of the house (former stable section) and a completely independent new building separate from the "front house." Otherwise, it would have only caused dissatisfaction and trouble in the long run.

The original house section, which was actually supposed to be renovated, remains as it is and will only be tackled once my parents are no longer around...

motto: good things take time

My parents experienced the same. They put money in their hands, fought for every screw, every stone. Then rebuilt after all.