Can you give some more details regarding your solid construction with KFN40 (especially wall structure)?
What specific questions do you have? KfW40 is not a problem. You just have to insulate properly, install suitable windows/doors, and use heat pumps/controlled residential ventilation/photovoltaics.
We just had two initial talks with local architects, and in both cases it was clearly stated that solid construction with KFN40 is practically not built in our area. The additional costs compared to the Building Energy Act would be so massive that in both cases (and these were not small architectural firms) there is practically no experience due to lack of demand.
Correct. It’s really not worth it. Either with QNG and subsidies or just as it fits. Because in the end, the energy consumption of both variants (55 and 40) is very close to each other. If you heat a typical single-family house 55 with 600€ per year and a single-family house 40 needs only 450€, it takes more than a lifetime to recoup maybe 10-15k extra costs. In doubt, you build with ETICS and then get annoyed about too thick walls. Or you switch to resol insulation for a thin ETICS structure, where the insulation material alone easily exceeds 100€ each.
P.S. I implemented a resol wall structure myself during renovation. I heat 120 sqm living space (formally KfW 55, but better insulated walls) with 350€ including hot water per year. So why should I try to achieve a 40 house? Better take care of external planning of your heating system, because that’s where much more is going on than in the wall structure between 40 and 55.