And KfW 40 is best achieved with a prefab house provider who specializes in and is familiar with it. For them, it's a home game and they offer a different price than an individual planner building stone by stone. Moreover, the wall is almost half a meter thick and other things are simply better solved with timber construction. Sometimes building materials are used that were developed for this purpose together with major manufacturers like e.g. Knauf.
Don't just claim and theoretically set up some calculations. Just go out of interest to a prefab house provider, have it calculated, and then put your ancillary costs and especially the steadily rising electricity costs alongside. And then say again it wouldn't be worth it... Alone the eliminated building stress is an enormous added value.
We also once had a conversation with this provider and they were rather in the category extremely overpriced.
That little insulation and such does not justify tens of thousands of euros in extra cost.
The technical brochure reads really nicely, theoretically, but all of that can be done the same and even better with a solid house. Only the wall structure differs. There xy indicates the compartment value. The overall value is between 0.12 and 0.13. That can also be achieved with 17.5 KS and 24 EPS 032. Just over 40 cm and still far from half a meter even with plaster. But it's simply totally uneconomical.
Just for fun I calculated the energy consumption of our planned KFW55 house: 3,500 kWh for heating plus an estimated 3,500 kWh for hot water. Makes 7,000 kWh. With a ground source heat pump with an annual performance factor of 5 this results in 1,400 kWh electric.
KFW40 has 21.4% better insulation than KFW55. That results in 2,750 kWh for heating and also 3,500 kWh for hot water. Together 6,250 kWh. Now you have an air heat pump with an annual performance factor of 3.8. That results in 1,645 kWh electric.
A KFW55 house with ground source heat consumes 15 percent less than a KFW40 almost passive house with air heat pump. This KFW label in the end is just pointless calculation.
Now calculated further, you would have a ground source heat pump. Then you would only consume 1,250 kWh electric. In this case you would actually save 3.12 EUR per month (25 cents as energy price without heat pump tariff). According to the current plan you rather pay 5.10 EUR on top compared to KFW55 with ground source heat.
KFW40, if you want it, is best achieved with corresponding insulation. Period. You don't need company xy for that. You also don't need timber construction. KS 17.5 and EPS032 20.0 are sufficient for KFW40. Completely.
And now the extreme case. We would build KFW70, so basically minimum standard. 21.4% more consumption, so 4,250 kWh and still 3,500 kWh for hot water. Makes 7,750 kWh. We install an air-water heat pump, and a terribly bad one at that - annual performance factor 3.5. We need 2,214 kWh and the price rises to 40 cents. Our monthly installment of 73.81 EUR will virtually drive us to ruin in 2036.
Edit: Of course I have now calculated our consumption up and down. Depending on your orientation, your house size, etc., different values result. What I actually wanted to point out: Whether KFW70 or KFW40 - we're just calculating peanuts here back and forth.