Mark airspace – build closed directly (approx. 6–7 m²)?

Well, let us move away from the legal-philosophical debate, in which, in my opinion, the positions have become clear, towards the probably originally practical background of the question or problem:

in our new building, the living space is calculated rather tightly. My architect said it was no problem to draw an air space (approx. 6–7 m²) in the building application in order to stay within the creditable living area, and then actually close it off in practice. He said this was common practice and normally not checked. It is a new development area. [ / ]
It is an attic where we are planning two dormers. As a result, we exceed the permissible living space overall. Therefore, the architect has drawn an air space in the attic, which is supposed to be used as a normal room.

So the original poster had a house designed whose upper floor/attic is not supposed to be a full floor according to the development plan. The architect (is he even one, or is the usual building application drafter of the general contractor just flattering called that here?) is overstrained in terms of competence with this task. In the planning result, in connection with two "dormers" (or even triggered by them?), the full-floor limit is exceeded. Now, good brainpower is expensive, and one looks for a cheap way out of the misery. In doing so, one believes to have found the philosopher’s stone by drawing a room (or part of it?) as supposedly floorless – unfortunately at the price of sin.

That the architect (or "architect") is a dishonorable fellow is probably a correct observation, but it does not really solve the problem. I throw the bold suggestion into the ring that the original poster should show the ground floor and upper floor/attic so that we could deal with the horse instead of the rider.

That the architect (or "architect") is a dishonorable fellow is probably an accurate statement,

I do not want my contribution to be interpreted that way and also want to avoid it coming across like that. When a business partner suggests acting at their own risk against regulations "because everyone does it that way," it causes a loss of trust in me. Whether this is justified remains unclear. That is simply not my culture. Taking a look at the situation as a drawing is certainly interesting and could be helpful for the OP.

The problem is just that if (almost) everyone is really doing it that way, you are the one who gets the short end of the stick if you don’t do it and thereby put yourself at a disadvantage.
Almost nobody here adheres to the regulations from the development plan, at least when it comes to the outdoor area, meaning compliance with the floor area ratio, mandatory planting or mandatory greening of garages and carports, etc.

Some neighbors even store their building materials on the public green strip; even after an inspection by the city, nothing happened there ;)
Then I started doing that too, so the stuff doesn’t block my driveway...

I don’t know what it looks like inside. But getting an inspection from the building authority for the interior of the house, I consider 99.5% impossible. They don’t even have enough time for inspecting the outdoor areas by drone or for timely processing of applications.
Neighbors can’t report you for something they don’t know about. So if you don’t invite them into the house and don’t tell them anything... how should they find out about an "airspace" that doesn’t exist?

And even if violations are discovered, what is supposed to happen? As long as the violation does not represent a danger, it will be dealt with by a fine, at worst a demolition will be ordered, provided it’s not disproportionate. That means a paved area that exceeds the floor area ratio will have to be removed, but a missing airspace will probably be settled by agreement. No one will tear down the house because of that.

Unfortunately, "blunt is trump" usually works.

And even if violations are detected, what is supposed to happen? As long as the violation does not pose a danger, it will be dismissed without a fine, at worst the dismantling will be ordered, unless it becomes disproportionate.

In my former place of residence, it was also like that once; everyone did whatever they wanted. At some point, employees discovered the usefulness of drones. Garages were demolished, swimming pools dismantled, conservatories removed, terraces turned into lawns. All of this was considered proportionate. The building authority was so annoyed by "blunt force is the key" that they launched a real mission. I do not know a single person who was able to successfully defend themselves against it. Legally, the situation was crystal clear.

I agree with you that it is extremely unlikely to attract attention inside the house. What makes me suspicious is that the architect knowingly submits a proposal that violates building regulations. Does he later also say to the craftsmen - here you can save something, the client won’t notice anyway? Does he check the cubic meters of excavated material during construction supervision similarly and let the earthworks contractor bill a few cubic meters more and a more expensive soil class? My trust would be shaken. If someone is ready to do something crappy, why should I be safe from him?

The naive idea behind this is that an architect who doesn't bring something like this into play simply doesn't do it. Here, he is cheating in favor of the clients. I would rather hope that he knows someone for the disposal of bad excavated soil instead of sticking millimeter-precisely to paragraphs... who doesn't know the story where clients weren't even allowed to use their excavation on their own property, while the municipality simply piled it up as a noise protection barrier to save disposal costs...

That building authorities go crazy like this is rather a rarity and there must have been massive violations if entire garages and pools have to be torn down to comply with building regulations. That can only be because these facilities were built outside the permissible building envelopes or the floor area ratio was so massively exceeded that it doesn't help to just change a paved path into gravel or the terrace into wood.

And it's outside, where with drones and appropriate software it is now very easy to check the floor area ratio.

I certainly don't believe that a complete house will be torn down or the statics massively interfered with because an air space was not executed as an air space. And as I said, you first have to stumble upon it. Neighbors don’t just hassle each other lightly; they have to be 110% sure that everything is okay on their side... and that is, at least in my neighborhood, definitely not the case ;)

The naive idea behind this is that an architect who does not bring such a thing into play does not do such a thing.

That is complete nonsense. Such a reverse conclusion is neither logical nor conclusive. Whoever calls for fraud is ready for fraud. Of course, this does not mean that someone who does not call for fraud is not ready for it.