Install controlled residential ventilation because of misinformation?

That's of course very annoying.

Nevertheless, you should check whether a/your legal expenses insurance covers this. Even the "star lawyer" only has limited time and inclination to take on an authority (unless you are the "star lawyer").

My insurance representative recently said that hardly any legal expenses insurance includes disputes with authorities in their offer – the idea being: "Better not to take on a giant, it just gets expensive."
I would actually, as the construction expert already wrote, first go with the letter and inquire about how things look. You can still shoot with the cannon later.

So, the matter has been resolved. They installed a so-called "Gartenhydrant". At first glance, it looks exactly the same as a normal hydrant. However, the firefighters can tell by the sign and then know the situation (the extraction volume is not sufficient for extinguishing). The sign will also be moved; the electricians will have to go past where it is now anyway, and then it will be closer to the street.

They are lucky...