Floor plan design single-family house with around 150 m²

Your window surfaces would also be sufficient for three-part windows everywhere (with mullions) without it becoming gloomy. The facade structure can handle both here: single or (with or without mullions) two-part. Thinking the other way around: the kitchen window door could cleverly be designed as a talking door, divided at the sill height of the window by the sofa.

Your window areas would also be sufficient for three-part windows everywhere (with mullions), without it becoming gloomy. The facade design can handle both: single or two-part (with or without mullions). Thinking the other way around: the kitchen window door could cleverly be designed as a French door, split at the sill height of the window by the sofa.

Interesting idea, I wouldn't have thought of such a door. It definitely looks good.

One more question about the eternal controversial topic of the interior wall on the upper floor: we now have to decide between [Kalksandstein] (11.5 cm) and drywall (double-layered on both sides, 60 mm insulation, so slightly better than usual). Our main concern is soundproofing; in theory, this drywall should even be better than the [Kalksandstein] wall (53 dB vs. 4x dB), but we have also heard from acquaintances with drywall that you can still hear quite a lot...

We are mainly concerned with soundproofing, in theory

There is no such thing as sound. In the attic, you have nothing above you anymore, so it essentially only concerns airborne sound by category. There are two widespread naive ideas about this: first, that the heavy wall would quasi-filter out vibrations through its inertia, and second, that its high density would slow down the sound. Both are rather quaint. Stiffness can also be achieved through profiling, so "bracing" is no less helpful than monolithic solidity. However, the wall alone does not a summer make; you can do a lot wrong at the component connections. In this sense, the worst thing would be to let the contractor build what they have less practice in. So don’t let yourself be unsettled by the idea that only calcium silicate bricks with left-handed lactic acids have enough fiber content—the drywall will not make you regret it; in my opinion, it is an equivalent alternative in practice. Having double cladding is already a safe bet. In everyday life, one listens with ears, not with calibrated measuring devices.

you won’t regret the drywall, in my opinion it is an equivalent alternative in practice. Using double layers is definitely not a bad idea.

We were also told that especially when pipes/cables (water/electricity) are installed in the drywall, the sound insulation deteriorates significantly compared to a sand-lime brick wall, and for example you can then hear the noise of running water much more. For example, we have the bathroom directly next to a room upstairs and the sink and shower are on the interior wall, here I fear that showering or turning on the faucet will be heard more in the adjacent room with drywall than with sand-lime brick...?

You will never get a wall with the sound of running water inside completely silent. Theoretically, you could make the walls uneven, but in practice, the builder usually has the routine with one or the other construction method, which suggests letting him choose only that way. So the same applies here in essence as I usually say about white and red bricklayers: let him do what he can, and don't try to reform him. One jot will always be missing from the hundred-percent dream house - if it is that last bit of sound optimization, so what.

But first, finish the floor plan decision; we will better clarify such details at the working plan stage.