KfW-70 standard not reached (bungalow)

On the topic of ventilation systems, you can ask 10 people and you will always hear different opinions. You have to clarify this issue for yourself. We will install one, also simply because we are both working and cannot ventilate as much as would be necessary every day. In the end, you decide what is important for you.

But if no one is there, no used air is generated either?

The topic is still ventilation systems. And the opinions are so different. One says it would be unnecessary if both are fully employed and therefore hardly at home. Acquaintances of mine have a ventilation system in their new build and are not satisfied at all, because the house is unnaturally warm. And you must not forget the maintenance costs.

So do you think a ventilation system is absolutely recommendable due to the (too) good insulation of today's new buildings?

Here is only the opinion of a layperson in construction, who has dealt a little with the topic over the past two years.

PRECISELY because both of you are rarely at home, the ventilation system makes sense.
The ventilation system, of course, serves you partly for supplying fresh air.
The more important reason, however, is the removal of moisture. Since you work in real estate and constantly deal with energy certificates, you surely also know that newly built houses are much better insulated than older buildings. This generally makes air exchange difficult. After the drying phase, the new building still contains quite a bit of moisture. In addition, moisture also appears in your "living environment." Showering, bathing, drying laundry, but especially you yourselves, e.g. when sleeping (google it). This moisture should be led out of the house because otherwise it condenses and there is a risk of mold.
It is recommended that the house (especially in the first years) be ventilated by airing it out 4-5 times daily. However, you work, which makes this impossible. Apart from the fact that window ventilation blows your heating energy directly outside again.

The house is unnaturally warm.
However, this is less due to the ventilation than more to the heating/heating type/heating setting. We had/have this problem too. Especially in the bedroom.
The ventilation only sucks up the already existing warm and used air, stores the heat and returns it to the supplied air. But it does not make anything (additionally) warm.
We were used from the old apartment to leaving the heating off in the bedroom and sleeping with the window open. Now – in the new house with underfloor heating – this does not work. The underfloor heating cannot simply be turned off just like that. All living rooms are connected with each other (expressed in a layman’s way) – there is always a heat exchange. Also, it bothers me to sleep with the window open while the heating is running (I do not have money like hay).
Here you can only try to work with various settings. We are still looking for the most suitable one.

But if no one is there, no used air is created, right?

Never come back from vacation and enjoyed the "scent" of the apartment/house? Besides the smells, you also have the issue of moisture... ... no one gets rid of that from the house either if no one is there!

But if no one is there, no stale air is produced, right?

Have you ever come back from vacation to an apartment or house that hasn’t been ventilated for 14 days? Then you know that the air smells stale even though no one is there. In my opinion, this is even worse in new houses than in older buildings.

For financial reasons, we also decided against a ventilation system. We usually ventilate the upper floor manually twice a day and the ground floor once a day (in addition to regularly opening the front door and patio door). This is especially necessary in the bedrooms on the upper floor (stale air) and in the bathroom (moisture after showering).

Interestingly, we don’t have major problems with humidity levels.
Ground floor between 40 and 50%
Upper floor between 50 and 60%

I am also an advocate of such a system for the reasons mentioned above in insulated new houses!

I am not exactly lazy and yet I knew before our house construction that I do not have the leisure to let the cold into the house twice a day. Nevertheless, we still have to do our rounds every day and manually move the blinds up and down. However, it stays warm.

And if you now write that you are only shortly before signing anyway, so still in the discovery and pricing phase, then it is admittedly dumb, but understandable to me if the contracting party is now calculating and presenting the result. Imagine if they just wave it off with "it'll be fine," calculate later, and then the mistrust and trouble arise between the parties.