Ventilation in the dressing room on the attic floor was forgotten

Sure, everyone makes mistakes sometimes, but our small list of errors is getting longer day by day.
These aren't even extreme things. Actually, "just" basics that every apprentice has to know.
Anyway, that doesn't matter here, the fact is, the ventilation was obviously forgotten there.

I have been able to gain quite some experience here and especially personally over the past years. And 99% of the time, these basics are forgotten, and the client gets really upset about it. Then people start gossiping that something like this just can't happen blah blah blah and what kind of botched companies these are. By the way, in my job I don't know that so many things go "wrong," but in construction this somehow seems to be normal.

Even if you sugarcoat the thing with clothing now, how does it look with the KFW compliance?
I think it is quite important for KFW40 that every room in the network is ventilated.

I don't know the details about that either, but I don't assume that one outlet makes that much difference. If it does, then either it has to be fixed or the financial damage will be paid by the developer.

What could a decentralized ventilation look like?

A hole is drilled into the wall and a corresponding device is installed. Google brings up quite a bit... In the end, you see a 15cm x 15cm plastic cap.

I don't want to go into detail about how the house construction proceeded and still is.

No one asked.

Plans were sometimes changed without informing us and

You must have the original plan...

I think it is quite important here with KFW40 that every room is ventilated in the network.

Not really. What are you afraid of, could happen? The bank won’t give you the KFW loan and will take house and home Or You will now consume twice as much energy as expected Or You will suffocate because the controlled residential ventilation does not supply fresh air, sorry, exhaust air.

Even if you sugarcoat the thing with clothing now, how does it look with KFW compliance?

What I can actually understand: it can smell. Not everything stored is freshly washed. But I already said: decentralized ventilation. You can have as many concerns as you want. There is a solution: decentralized ventilation.

As far as the KFW is concerned. That's just calculations. If your energy consultant calculates Kfw 40, it doesn't matter how. The main thing is to achieve it. Right?

From the overall picture of your description (almost identical house in the second construction phase and the like) I deduce that this is actually about a developer ...

DEKRA accepted the house and we are still living in a 4/5 construction site.

... therefore I suspect the expert organization accompanies the construction as a quality controller commissioned by the developer – however, I cannot reconcile the terms "accepted" and "4/5 construction site," they sound contradictory to me.

But who decides whether a ventilation system is installed there or not?

In the plan I approved, no ventilations are generally indicated.

As the buyer, you decide WHAT the developer builds for you. Two contractual bases or attachments for this – namely the construction and service description and the plans – should say the same thing or at least not contradict each other. If the construction service description says "central controlled residential ventilation, but only in living areas," the dressing room is excluded. If construction service description and plan say "central controlled residential ventilation, also in storage rooms," it must be clarified whether the connection of the room or only the outlet on the duct was forgotten – and in the former case, the appropriate solution is to retrofit the controlled residential ventilation in the dressing room in a decentralized manner (which is not a big deal, who really needs heat recovery in the dressing room). Is there also no over-/underflow of the door to the dressing room and are the supply/exhaust outlets in the bedroom too far away to serve the dressing room?
In general, for the developer it applies even more strictly than for the general contractor: "what is not included is at sea / in God's hands."