KFW40 or better in solid construction, possible without additional insulation?

Which standard could a solid house become with:
36.5 cm Ytong
Controlled residential ventilation
District heating with primary factor 0

... Are we already at KFW55?

Yes, that depends on the interpretation, but basically you are right. I didn't have the number at hand but it's more like 1200. That's why there is also the "fun" at the end.

The 450 kWh can be used as a delta. That also fits with the photovoltaic system. My electricity provider sent me an increase on Friday for 01.01.2020. It was already over 30.5 cents. Tell me someone that photovoltaics aren't worth it.

Amortization under 5 years and I'm in. Until then, I don't think much of it for my investment strategy. But with photovoltaics, you can't go wrong if you want to.

Amortization under 5 years and I'm in. Until then, I consider it of little importance for my investment strategy.

But you can't go wrong with photovoltaics if you want to.

I've never seen photovoltaics as an investment with a payback period of more than 11 years anyway.

What standard could a solid house achieve with:
36.5 cm Ytong
Controlled residential ventilation
District heating with primary factor 0

... Are we already at KFW55?

If Ytong has a lambda or U-value of 0.20, then yes.

My preferred wall structure would be [Kalksandstein], [Mineralwolle], air, [Klinker]. It shades itself in summer, has good sound insulation, and dries out on its own. Since you want to do a lot yourself anyway, I would recommend thicker insulation for the roof and the floor. Both then done in [EL]. The additional financial cost for the materials is manageable. If you also have a controlled residential ventilation system, your heating load will drop below 5 kW. Depending on the region, an air-to-water heat pump will easily suffice. There is also potential for savings here, depending on your "risk appetite." Photovoltaics would always be included for me, but financed separately.