Two-family house to one-family house with an apartment

Good day,

I or rather we are currently considering buying an existing property.

Who is we?
My wife, my 2 children and I as well as my parents-in-law (58/60).

It is a classic two-family house built in 1980.
Living area: 236 m² Plot: 643
Ground floor: 136 m²
Upper floor: 100 m²
Construction method: Solid
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Number of floors: 1
Construction: Open
Floor area ratio: 0.4
Plot ratio: 0.5
###

Now I am thinking back and forth about how the floor plan could be redesigned so that my parents-in-law (who are supposed to get a granny flat) can manage, but we still have enough space for us and the children. Classic one above the other would be easiest, but it would not be our dream.

I am considering possibly connecting the right half of the ground floor and upper floor with a new staircase?!

Space requirement for parents-in-law is about 80 m²
Accordingly, 156 m² would remain for us

We have not yet consulted a structural engineer.
My primary concern is to get general ideas first (hope that is okay)

Maybe someone here from the forum can help me?
I am grateful for any suggestions.

Thank you

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Why do you want to change something there? It is a two-family house with two usable separated apartments... ideal for you

I don’t quite understand your problem. Nor your proposed solution: having 100 m2, they want 80. If you add an extra stairwell, you lose 10 m2 again—both upstairs AND downstairs. And another 50 to 80 thousand euros on top.

Why do the parents want to live downstairs, what is the intention behind that? Don’t you like their impact sound? Do they want a garden on ground level? Or...?