We are building a KfW55 house (this year) without controlled residential ventilation. So it is possible. For us, it was a question of price (easily about €12,000). Our wall construction (timber frame prefab) meets the KfW40 insulation values (already standard with our general contractor). Therefore, controlled residential ventilation is not needed. The KfW application also went through without problems. Only in the bathroom will we install a decentralized device. Why? Because that’s where the shower is, the washing machine/dryer are located, and the laundry hangs there. To be safe, we are installing a device there. Interestingly, every salesperson (the house manufacturer) advised us against controlled residential ventilation. Even a civil engineer in the family did. Apparently, the majority of houses are now being sold again without controlled residential ventilation (statement from 2 manufacturers). Whether that is true, I cannot judge. I was also surprised by that.
Now honestly, who has had controlled residential ventilation installed, whether central or decentralized, because of the KFW subsidy? Apart from those who want the higher subsidy now, everyone else has the system for comfort or fear of mold. For me, it is the second. We simply cannot ventilate enough to avoid something like that. Previously, we had a completely renovated apartment for first occupancy with ETICS. From the first year on, there was mold spotting even though, from our point of view, we ventilated a lot and correctly, meaning no tilted windows.