Is it worth paying the extra cost for KFW55 or not?

Thanks for the answers so far.

Why do we want an air-to-water heat pump?

I hadn't questioned it until now because we also wanted KfW55, and no radiators, but underfloor heating, and the air-to-water heat pump is so common and advertised.
Otherwise, a gas boiler with condensing technology would be included in the base price.
Additional cost of the air-to-water heat pump €6900 compared to the gas boiler.

Then the gas connection costs also disappear. Unless you absolutely want a gas stove.

Regarding ventilation: We were thinking of an intake/exhaust fan in the large bathroom and two alternately operating fans in the large living/dining area. Would that also be okay? Window frame fans and exhaust fans in the bathrooms are good in that the fans do not cause disturbance. I would just have to come to terms with the concept of making a tight house leaky again.

The KfW55 surcharge results according to the offer from better insulation of the roof (additional external roof insulation), the exterior wall, the roller shutters, windows, the basement (perimeter and floor insulation)

Additional roof insulation and the windows determine the price. Roller shutter boxes are peanuts, the basement, wall, and perimeter are already there and just need to be made thicker. I would leave it at the surcharges; it's not worth it. The program conditions have simply become too bad for that. We just financed 20 years at just under 2%, the KfW offers a great 2.6%. And thanks to the development plan, we are actually building to KfW 55 quality (almost 40), but nowadays no one wants their money! The repayment grant doesn't change that, and the appraiser is twice as expensive (despite separate funding). It's a pity, really.

I would leave the surcharges as they are and just take the Energy Saving Ordinance with gas. How much do you save with KFW 55? €150 per year? Even with rising prices, you can heat for several years with that, and you must not forget that you first have to finance the additional costs yourself. You also have to factor in the extra interest (KFW then basically more expensive than at the bank).

I would somehow leave out the reciprocal fans in the living area, at least for us that was rubbish.

Gas condensing boiler Junkers is installed with us. House warm, easy operation, I guess about 600 euros costs per year. But that still needs to be confirmed, so far just estimated. Window rebate ventilator. Well, every ventilation makes something leaky. If everything is tight, nothing ventilates. I don't want to miss that. Always good air from outside in the room. The Helios exhaust air rotor system sucks air out of the house and blows it out, fresh air flows in through the window rebates. Simple and good and costs only about 1000 euros gross. Keep it simple, keep it cheap and rugged and reliable. What isn't there can't break, doesn't need maintenance, causes no worries. All of England thinks like that, and so far they have always won the war, we always only second.[emoji41]