Guest bathroom planning

If I'm not mistaken, washbasins are usually hung at a height of about 80-85cm. So it wouldn't be standing in the window.

You don't need a pre-wall installation for the washbasin. However, for aesthetic reasons, I would still do it continuously. Then you'll also have a shelf across the entire width of the wall.

Ah ok thanks, then I need to coordinate that again with the planner. Regarding the [Vorwand] you are right for optical reasons, but I don't know if it might make sense in our case to do without it since you gain more depth for the sink. The [Vorwand] is planned with 20 cm as I see. If the sink is on the left, you have more space with the door, which I would prefer not to have open outwards.

How big should the sink be, that is, how deep? And who will use this WC how often?

We are still flexible in the planning regarding the sink. If I understand correctly, if pre-installation is done behind the sink, we have 20 cm until the window starts. But according to my research, there won't be a sink with only 20 cm depth, or a sink is not allowed to be too small. We were thinking of a width of about 45 cm for the sink, or maybe smaller depending on your opinion. One has to weigh the options; if the sink could be planned on the right, then there will be more space in the WC, also regarding the opening door. This is purely a guest WC, which of course we will also use when we need to use the toilet on the ground floor.

The proportions can be easily recreated at home. In my opinion, it fits with the sink at the bottom of the plan, the toilet at the top of the plan, both on the same (front) wall.

But a washbasin with only 20cm depth will not exist according to my research, or a basin is not allowed to be too small. For the width of the washbasin, we thought of about 45cm, possibly smaller.

Calculate with a washbasin size of 48 (50) x 38 (40). It shouldn't be smaller. And I assume that the parapet height is rather higher than the usual 85cm? Then it doesn't matter if the two elements collide in the plan, in 3D they don't ;) If necessary, move the window a little towards the wardrobe window :D