180 sqm non-replacement house, best adaptation to the plot?

Piano and fireplace together will be difficult for the piano.

Sorry, that was misleadingly expressed by me: either there is a fireplace with surrounding seating at the height of the dining room floor OR the piano. Having both at the same time would indeed quickly reduce the piano to its calorific value.

I think the living room lacks a bit of "retreat space"

Yes, I consider that a very important point, thank you for that, the cozy niche is indeed hardly possible there. Due to the upper floor (two rooms next to each other), the living room has become wider than originally planned, and this has reinforced it. We need to let that sink in. We also need to tackle more sharply the question of where the then 17-year-old “puberty creature” will or should have their room in five years.

If you don't have enough space downstairs, consider a different room layout. Cooking/Eating/Child on one level Living/Parents on another. Your child is a T-ager until you move in. They no longer have to live on the same level as Mom and Dad. Living could then truly be a retreat, with open gable, gallery, etc.

Because of the upper floor (two rooms side by side), the living room has become wider

You can also plan with an overhang ;)

You can also plan with an overhang ;)

Definitely, you mean something like this ;) ?
I generally like it a lot, and it's also planned for tomorrow’s draft 3, but if the spatial requirement on the ground floor > upper floor, then rather a "underhang" (= terrace on the 1st upper floor)... Or both, like in the attachment... does it look like that?

Unfortunately it increases the plot ratio, or – if the plot ratio is already maxed out – it only makes negotiations with the building authority slightly more difficult... :p...

Unfortunately increases the floor area ratio,

True. If it is equal to or greater than 50 cm. But it doesn't fit your chamfered part anyway.

But that doesn't fit your beveled part anyway.

Agreement.