Planning controlled residential ventilation via local heating / sanitary company

Painted above with lime dispersion and below with silicate dispersion. So, yes and no. But I can't really imagine it either. Except that my gypsum plaster is already behaving "strangely" from my perspective. In the meantime, I also suspected sintering, but with the spray test no drops rolled down.

...maybe chip off a piece of plaster in the area of the baseboards and measure the moisture underneath?

Yes, that would be one approach. I also have somewhere one of those capacitive moisture meters.

our house (202 sqm on 2 full floors)

You haven’t shown that in this forum yet – but you should (as a basis for consultation for this question, not to put it up for discussion – unless it is still up for discussion).

Last house central controlled residential ventilation – planned by the heating engineer (whatever about that – it doesn’t matter).

You are also a remodeler, so that makes sense.

Basically, I would always have a controlled residential ventilation system planned directly in new builds, i.e. by the architect of the new building project (but get a “second opinion” consultation from a specialist in controlled residential ventilation, because not all planners are proficient in this, myself explicitly included). What would be the biggest nonsense – hence my suggestion, even if this is “only about the controlled residential ventilation” to show the whole house – would be: although the house as a new build only “exists” as a plan, to retrofit a “retrofit solution” as if you had bought it as a bare shell. What can be planned should be planned early. A controlled residential ventilation system is a crucial element that must be taken into account in the overall design. And it is a fundamental decision that can possibly influence the choice of architect: whoever denies the immaculate conception definitely cannot become a Catholic priest. At least one central controlled residential ventilation system – which I would prefer if at all (with heat recovery and/or humidity control even absolutely) – significantly affects the construction of ceilings and floors. If you decide for it, the general rule “better late than never” also applies here – but you should be aware that a later point in time is rather unfavorable.

As my predecessor says, a controlled residential ventilation system is a fundamental decision that must be made before any planning, as it can affect ceiling thickness, structural integrity, and room heights. However, I do not see any indication here that the original poster wants to install this afterwards.

Regarding the question in the original post:

I planned our ventilation system myself using the tool from Helios (?). However, it mainly calculates how much air each room needs and how many ducts/vents you require. The program does not tell you where each duct should go. But that is not rocket science. The ventilation unit can be (if necessary) replaced, but not the ducts since these usually disappear in the ceiling or floor.

As my predecessor said, a controlled residential ventilation system is a fundamental decision that must be made before any planning, as it can potentially change ceiling thickness, structural engineering, and room heights. However, I do not read any indication here that the OP wants to retrofit this later.

Whether the house is already finished or "only" in the planning stage: I have warned against retrofitting a controlled residential ventilation system before any such attempt. I did this only because of abstract fallen dice, which you have now thankfully named in concrete examples. Not every builder is in the relaxed situation of and has ensured, by increasing by one row of bricks, "not to die from 'Morbus Crocodile Tears'" because of 6 cm more ceiling suspension ;-)

after much back and forth, I have almost convinced my wife that we need a controlled residential ventilation system.

... at least I did not read this as an indication that convincing the spouse was or is to be done before entering the design planning. For example, if — which is why I suggested showing the whole house — a change in story height or floor structure due to its correlation with the stairs would disrupt the floor plan or if the ceiling were already ordered, this would add a special twist to the freedom of considerations. That is always the problem with individual screw-questions: to give therapy recommendations without knowing the stage.