Construction cost planning KFW 55 realistic???

I could imagine that 265k€ will be very tight. Our KfW55 bungalow (165m², monolithic, ground-source heat pump) is around 275k€ in Brandenburg.

That is then really very cheap, even in Brandenburg. Although of course you have to ask about the fittings. If you only install good and inexpensive bathrooms, cheap tiles or laminate, and refrain from "fancy" things like stainless steel Schlüter strips, ceiling spots, electric shutters, surcharges here and there, it might just be enough, but you have to be very disciplined and nothing can go wrong.