How does one correctly balance between the Building Energy Act and KfW 55, 40, and 40 Plus?

Exactly, but 25% is not 60%.

That's not what I mean either.
The heating costs you have can be based on the Energy Saving Ordinance or as a passive house – I paid about €250 last year for district heating in the 60s apartment outside the Energy Saving Ordinance (albeit with energy measures). However, I also have neighbors who heat the same apartment for €1,600. It depends on the user. The building's energy standard only provides a possible framework.

It's not about who has how much heating costs and how they are influenced by user behavior, but about the question of which standard the house should be planned according to... and there are professionals for that. Houses are too individual for general statements. At the latest in the building application, the information has to be provided anyway - I advocate involving the energy consultant already in the planning stage. H

We would like to independently review the advantages and disadvantages once again and wonder which points should be taken into account here. From our point of view, these are the following:

[*]Financial considerations
- Repayment subsidies for KfW55 are generally about as high as the additional construction costs, so this is more or less cost-neutral
- Energy-efficient construction saves heating costs in the long run, so it should be cheaper in the long term to build energy-efficiently
- KfW loans currently have little relevance, as similar or even better conditions are available from banks.

If the budget allows for a higher standard, you will have lower energy costs in the long term. This is a simple amortization calculation.

Indoor climate
- Too much insulation harms the room climate (We only know this from friends, but we are not sure if this is always the case)

Strong insulation does not inherently harm the indoor climate. Material, heating, and ventilation are decisive here. However, I have also had the very personal experience that in many new houses you get the feeling of sitting in a Tupperware box. Conclusion: You do not only decide on meeting the standard, but also on the execution. This is price-relevant regardless of which KfW standard you choose.

Land utilization
- With a thermal composite system, KfW55 can be achieved with thinner walls, which would allow about 10 sqm more living space on our narrow plot.

Discuss the special requirement of "thin walls for more living space" with the architect. Here I would follow the design. Bigger is not always more usable.

Value retention
- Are there already any empirical values regarding the value retention of the different construction methods?

The value retention of a house in terms of quality depends less on the KfW standard than on the overall craftsmanship and material execution. The value retention of a house when selling depends on the market situation. If there is a large supply, "features" help especially well to find a buyer and achieve a decent price. If there is high demand, the fact that a house can be sold is more important than its characteristics.

Regarding the question of which energy efficiency level one should build with: If you are young, 25 -> as efficient as possible If you are around 40 -> it’s not really worth it anymore, better to invest little and be done with loans before retirement. If you are even older ... 50 -> forget about any additional insulation. Expensive thermal insulation only pays off over time, I built young and therefore invested in KFW55. If energy is three times more expensive by retirement, I can live with that. If I didn’t have KFW55, the 3x more expensive would quickly turn into 6x as much as today. Unbearable

You assume that this pays off significantly. But it hardly does. The difference between 45 kWh/sqm (Energy Saving Ordinance) and 25 kWh/sqm (KfW40) sounds like much more than it actually is. If you compare this to the fact that in the 70s houses were still built with 300 kWh heating demand and a decent unrenovated old building is around 160-180 kWh, you realize how good houses are today and that hardly any savings can be made on heating regardless of the standard. At the same time, there is little reason to worry about heating costs getting out of control in a house built to the Energy Saving Ordinance standard. And most of us won’t be heating with gas in retirement anymore. After oil burners, gas will be next to be phased out in the coming years, for the sake of climate protection. Therefore, instead of questionable amortization calculations, it's better to make sure that the heating system generally works efficiently with heat pumps or similar. Choosing radiators instead of underfloor heating to save costs will most likely backfire in 15 years.

So if I did anything right in our project, and I mean really right, it was the decision to involve a sensible energy consultant (in our case an engineering office / expert) early on.

Simple question, but how did you find the right cat in the bag? Trusted your construction partner? I looked at the 4-page list of energy efficiency experts in our area and always wavered between a "solid craftsman" and "rather a fancy architect," switching back and forth.

Some things that went well can be better explained in hindsight than in foresight. We all build more or less as laypersons and make decisions out of relative uncertainty. We have found it better to "examine" people rather than prices and to place trust in them. However, regardless of our experience, this can also lead to terrible failures. Others have taken care of watertight contracts and attribute the success to that. For others, this method failed miserably. We are all the authors of our experience reports and construct success stories and causal relationships backwards. Building is always associated with risks. The responsibility for decisions lies with the clients. Freedom always includes accepting the consequences of one's actions - regardless of their foreseeability or any potential issues of "blame."