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

Let's say: you either need thicker insulation – which will also eventually reach its limits, especially with monolithic-massive construction – OR more efficient technology. However, you have to swallow a toad, ideally the one that is cheaper. And if you don’t want a thicker (or a completely different one that costs less for a given insulation performance at the desired additional delta) wall structure, you will only be able to adjust the technology.

It’s about the primary energy demand related to the reference building. 1 kWh of heat from oil and gas "costs" about 1.1 or 1.2 kWh primary energy, or solar energy close to zero primary energy. There is a lot of politics behind this factor as well. That means theoretically you could heat a relatively thin envelope with a seasonal storage solar thermal system and it would be okay.

But at some point, this additional effort just won’t be acceptable anymore. Except with true believers.

it really wasn’t. We had 2 timber frame builders with KFW40 in comparison, one was more expensive, one was a bit cheaper. That surprised us too. Whether something has changed since 16/17 I don’t know.
It’s important that if you want KFW 40 or Passive House, you find a company where that is the standard.
For your information, it looked different with KFW 55.

I think you will most likely be happy with a timber frame or log house. Insulate it as much as possible and consider the certification. It’s not automatically worth it.

To contribute something more to the initial question: We partly have walls made of 42.5 cm bricks + plaster and 30 cm bricks + mineral wool + wood facade (ventilated). The area with the wood facade was about 25% more expensive but also has a 25% better insulation value (if the same insulation value had been used, it would have been only slightly cheaper).

That's exciting!

Why did you build such a mix?

On the one hand: brick+insulation+wood was cheaper than brick+wood with the same U-value. On the other hand, there were constructive reasons to avoid thermal bridges in the transition to the parapet/flat roof.