Sorry to interfere,
... but at the latest after the statement ...
In any case, such a system initially involves an investment volume of around €10,000, and that is simply not financially feasible ...
it ultimately seems to be just about persuading oneself about the obviously identified possible problems ...
My personal opinion:
Anyone who today cannot afford a controlled residential ventilation system costing €9,000 should rather continue to rent.
(There are truly more than enough forced auctions ... !)
Anyone who believes that their previous ventilation behavior is sufficient for the object mentioned should better obtain comprehensive advice beforehand.
Anyone building with a developer should know what they are getting into.
What has been somewhat neglected in the previous discussion is the
airtightness problem of the heat-exchanging building envelope.
Anyone who believes that they are acquiring a sufficiently airtight house just because perhaps an air exchange rate is documented that is below the Energy Saving Ordinance limit (for "window ventilation," that is 3 complete, mostly unwanted air changes per hour) should be aware that permanent airtightness (in principle lasting a lifetime) is assumed here ...
Making a residential building "fit" only for the blower door test (or for handover/acceptance) can basically be done by any painter!
Anyone who believes that in practice the detailed planning of the airtightness layer required by DIN 4108-7, an analogous professional installation observing all mandatory framework conditions, and their control (including documentation) takes place should keep dreaming ... or ask the builder the right questions!
The builder also calculates.
Why should he recommend a ventilation system?
The resulting profit for him is certainly not in proportion to the effort he has to make to achieve the then applicable 1.5 air changes per hour (perhaps also a measured result to be verified by a truly independent inspector).
He will gladly suppress his liability problem at this point because the legal route takes time ... and hardly concerns him in the end.
(By the way, my test certificates basically state regardless of the measured values: ..." The measurement result does not exclude hidden defects." ... Why do you think?)
Determining the cost impacts resulting from such defects is likely far more difficult than the already mentioned economic comparisons ... unless you simply calculate the demolition costs right away.