Heating and ventilation system KFW40 house

Do you build a solid house or a prefabricated house? Why actually Kfw 40 and not 40+, 55, or Energy Saving Ordinance?

Solar only makes sense in connection with gas. With a solid house, KFW 55 with heat pump and controlled residential ventilation can be worthwhile, KFW 40 probably not.

If I could, I would build a house according to the Energy Saving Ordinance with a gas condensing boiler. If gas is not an option, then a ground source heat pump would be my favorite. Alternatively, if money is an issue, an air-to-water heat pump. In any case, with controlled ventilation with heat and moisture recovery. For the sake of comfort.

Photovoltaics must be calculated ... The effort alone puts me off.

Hello ares83 and Bieber0815,
thank you for your answers and opinions. @ ares83: We are building solidly. I will now consider your ideas/opinions and then get back to you.
Thanks and best regards,
Alex