Single-family house 160m2 with basement, 500m2 plot

Yes, that's exactly what I meant. Then two equally sized offices will be created, and the fitness room would lose that pointless little corner. However, the offices are quite small if they are also supposed to serve as guest rooms. An alternative would be as shown in the attachment - with a separate laundry cellar. There, you could plan a laundry chute from your bathroom. Due to the garage, no window is possible there. But if you have a controlled residential ventilation system, you don't need it either. For example, we have omitted windows in storage rooms, technical rooms, and laundry cellars, and it works flawlessly. The reason: I hate light wells, partly because they complicate terrace and garden design, and partly because you have to clean them, and I have arachnophobia. Oh yes, and if you really want to work there daily, definitely make large windows and a slope as already recommended. In my opinion, a staircase is not necessary.

Yes, controlled residential ventilation or decentralized or none at all is also not clear to us yet. I think it only works with controlled residential ventilation, right?

Is there a good thread on the topic of ventilation here or on the internet? I was in favor of ventilation, but the heating engineer is currently building his own house and is not installing one because he calls it a "virus spreader." That makes you suspicious...

Yes, controlled residential ventilation or decentralized or even none is also not clear to us yet. I think that only works with controlled residential ventilation, right?

In new buildings, I would never consider decentralized. Always central.

Is there a good thread here or on the internet about ventilation? I was pro ventilation, but the heating engineer is currently building his own house and is not installing one, as he calls it a "virus spreader." That does make you suspicious...

...mmh good question. The whole internet is not all that big now...

Besides prefabricated house/solid house, aerated concrete or Poroton, heat pump or gas heating, it certainly belongs to the most common questions and threads on the net.
Whether they are good though... is another question.

Regarding your heating engineer... well, what can you say... there are some who are quite special candidates. It’s typically not his "home trade," just like heat pumps usually aren’t either. Many come rather from the “I make fire” business.

Search through this forum and through the net. I can also recommend the pink-colored forum for that.

What you will read... there are many who are against ventilation. They are usually the ones who don’t have one. Of those who do have one, you will hardly find anyone who doesn’t praise it and find it great.
The thing does not spread viruses any more than it gets everything dirty. It’s not economical but a very big comfort. You have to be clear in a new building that you have to think about the ventilation concept. The houses are so tight that you have to ventilate every room regularly. That means opening windows several times a day.
You don’t have that problem with controlled residential ventilation. It just costs you depending on the situation about 12-15k.

We have continued planning now. Next week it should continue.

I think the covered corner is not really usable because of the post. I will ask if it can be done without the post. I laid it out in the garden and placed a table with 6 chairs. It is very tight.

A wardrobe for 5 people is too small. I will ask to make the guest bathroom as small as possible.

The dressing room is actually quite nice since we never really get up at the same time and everyone has their peace. But it will surely be torn apart here immediately.

The garage is now definitely big enough for one car and bicycles on the side. I would even make the garden shed extension a bit longer.

Would it help to use a platform staircase? Does it save space in the hallway?

What do you think?

Do you think it makes sense to swap the parents' bathroom and bedroom? Water above this bay window?

Why do you want to swap? The arrangement seems fine to me as it is.