Single-family house 140m² KfW efficiency house 55 achievable through insulation and air-to-water heat pump?

For the aforementioned reasons, I had therefore also said that you first need to recognize your individual needs. KFW can be implemented through all sorts of things that may be more or less meaningful for your living situation there. The "problem" is that with a GI you have little or no influence on the execution, so you should definitely clarify this in advance with the GU, for example, because otherwise you will get what suits the heating engineer but may have nothing to do with your expectations. Then you can run around the switch like crazy afterwards if, for example, the spacing or the general installation of the underfloor heating is poor; I currently have something like this here. I think you should actually read a lot here to first find out what type you are :D :D, that is, which kind of living you are sympathetic to. Everyone likes it differently and you can find that here to read up on. Heating is also only one part of living. It is best if you post your entire project here completely and from the following discussions you will then gain more clarity and also security. I would definitely only sign after an appropriate planning period, changes afterwards are expensive or often no longer possible; unnecessary haste or rush usually affects the result in construction! Therefore, it also makes sense to find a suitable energy consultant who understands your project and your wishes; then he may also cost more. It is like sometimes with the architect, who through clever planning alone can pay for himself.

We have relatively few wishes & ideas! We want a roof over our heads that is divided according to our needs & can be heated economically. No ventilation system, photovoltaics, air conditioning, etc... unless necessary. In my opinion, the simplest way to the goal is also the best. As long as living rooms are at 21° and bedrooms around 18°, I’m relatively indifferent about the rest. If this can be realized relatively economically through optimal installation of the underfloor heating, I’m in :D

Hm....I know this way of thinking and find it fundamentally likeable.
Nevertheless, in my opinion it makes a lot of sense to think about many things or to evaluate their sense/nonsense only after you have dealt with them.
It is by no means the case that if you worry less or not at all, your building will necessarily be cheaper and if I can arrange my roof over my head according to my wishes, why not, because your house will also cost a lot of money. On the contrary, it is even the case that you may be able to reduce some costs if you have dealt with certain things, because then you can sometimes say no when you are repeatedly told "you should definitely do/have this."

As long as living rooms have 21° and bedrooms about 18°, I don't care much about the rest. If this can be realized relatively economically by optimal installation of underfloor heating, then I'm in.

Most heating costs you save through your own behavior, you should also like the type of heat generation and effect. There is no general model but somehow you can live everywhere, then I would actually just look for the cheapest on the market; I mean that seriously, because you can also choose a house like that.
Therefore, I am a little "surprised" that you are practically without any demands but definitely want to build Kfw55. That demand would come way down on my list based on what I read.

From that perspective, I’m a bit "surprised" that you basically have no requirements at all but definitely want to build Kfw55 o_O

For us, it was relatively indifferent whether it would be an efficiency house or not. Since the general contractor (GU) normally builds Kfw55 as standard, we just accepted it. Or rather, not implementing it as KFW55 would have saved less than the KFW subsidy.
I do engage with the topics, hence my various contributions in this forum. When something seems off to me, I turn to the experts here. :) Ultimately, you have to trust the general contractor somehow, otherwise I wouldn't have needed a GU and would have just contracted the trades myself.