Is it sensible to retrofit a decentralized ventilation system?

Sure, it depends on who wants to play along, how, and where. I'm just thinking out loud.

Thank you for your messages

Unfortunately, I am not very familiar with the topics and am currently trying to educate myself.

Yes, we initially tried to become the builder with a prefabricated house.

Due to the lack of land and extreme prices, it became a developer new-build area. If we had not decided on the developer project, we would still be searching.

I first wanted to ask if a decentralized ventilation system would even make sense.

Is it even allowed to make so many core drills and install air conditioning in a KFW 55 house and create weak points in the insulation?

Is it even allowed to make so many core drillings as well as install air conditioning in a kfw 55 house and create weak points in the insulation?

The question is: Who pays?

I wanted to first ask whether a decentralized ventilation system would even make sense.

These or slot ventilators are your remaining alternatives. From my point of view, the decentralized version of a controlled residential ventilation system only makes sense for selected rooms - for most or even all, I consider the central variant to be more economical, but that won't bother us here since the BT probably wouldn't go along with it. However, it is – even with heat recovery – only a ventilation system, not an air conditioning system.

Is it even allowed to make so many core drillings as well as install air conditioning in a KfW 55 house and create weak points in the insulation?

I think you will get into trouble if you use a KfW loan or other subsidies for your KfW 55 house. Otherwise, refrain from doing so.

Honestly: just put a decentralized device in the bathroom and if you choose a quiet one, one in the bedrooms. You won't die from manually ventilating your KfW 55 house. In our construction area, out of 12 houses, only 2 have central ventilation and two decentralized, and the rest still live there. I am clearly in favor of central ventilation but if it doesn't work, it doesn't work.