Construction costs are currently skyrocketing

In your documents from the KfW, there should be something about a 10-year deterioration prohibition. In other words: even a core drilling and exhaust hood would be illegal. Whether anyone actually checks this is another question, of course. It is really not sensible anyway. You build an airtight, highly insulated house with great effort and then blow 500 cubic meters per hour out with the exhaust hood. This then also raises the question of where this volume of air can actually flow in from. There are now also good recirculation hoods.

Your documents from KfW should mention something about a 10-year deterioration ban. In other words: even a core drilling and an exhaust hood would be illegal. Whether anyone actually checks that is, of course, another question. It really doesn’t make much sense anyway. You build an airtight, highly insulated house with a lot of effort and then blow 500 cubic meters per hour out with the exhaust hood. Which also raises the question of where that volume of air could even come in. Nowadays, there are also good recirculation hoods

Thanks already. We have installed a really cheap recirculation hood in our rental apartment, maybe that’s the reason. Which ones are recommended? Bora system? If we nevertheless make changes on our own, do I then lose the warranty from the developer?

Developer means you receive the house in finished condition. With all confirmations of the ordered features, including [KfW 55] condition. If you then change something on your house, that is your business. However, the warranty for airtightness is likely gone if you make holes in the walls.

Thanks already.
We have installed a really cheap recirculation hood in our rental apartment, maybe that's the reason.
Which ones are recommended? Bora system?

If we make changes ourselves, do I then lose the builder's warranty?

We had Bora recirculation in the first house, combined with a controlled residential ventilation system. I would do it again anytime. No deposited grease like you know from cheap hoods, nowhere.
We are building it exactly the same way in the new house again.

:eek: my phone types too slowly. ... you are damaging the building envelope and the airtightness is gone for the BT. Instead of the Bora, we have one more empty cabinet for us, a recirculating hood, and a surface induction cooktop with 6 cooking zones (from the brand manufacturer)

We have an extractor fan in the wall cabinet. Miele with 12-fold filter, has so far been able to extract everything well, even when cooking ground meat, and there is no smell. Previously, in the apartment, we only had interior walls in the kitchen and a cheap recirculation hood, where most of the air passed by at the front. Whether to vent upwards or downwards mainly depends on the rest of the kitchen. If you are putting wall cabinets above anyway, it is also good to vent upwards. With an island or peninsula, rather downwards.

The builder will, of course, blame all problems on your core drilling if in doubt.