Toilet window in the guest WC next to the entrance door - is it now a no-go?

You can do that, but you don't have to. With ventilation systems, you can create absolutely soundproof rooms perfectly.

The statement surprised me as well; we've never heard anything about a slit beneath the doors...

The doors are often simply 0.5-1cm shorter to ensure the overflow of the individual rooms.

Zehnder and Tecalor even explicitly point this out.

However, you can also mill out the door frames and leave out the seal on top, then you don't have to shorten the door leaf so visually unappealing...

Our door builder wanted to talk us into installing a ventilation grille for the heating room *lol nice try

Since most bathroom windows are permanently tilted open outside the heating season, noises and odors can unpleasantly affect the residents, visitors, package deliverers, and mail carriers at the front door, causing embarrassment, he said. I myself also, of course, have the bathroom window next to the front door - ggg Now we can return to more relevant topics.

Ventilation system... nothing is permanently tilted...

So no one sees that, with us the doors are probably 0.7mm shorter than the frames.