Economic efficiency construction costs vs heating costs

Your generalization is an even bigger nonsense and has a barroom level.

Next thought .... [Aufsparrendämmung]

These individual measures are pretty useless. A house needs a coherent energy concept. That means setting the desired standard and then having it worked out by a professional.

If you really want to burn the money in advance, then probably a controlled residential ventilation system is still the most reasonable. Economically, you won’t gain anything from it, but at least you have an apparent benefit in comfort.

Otherwise, in my opinion, the by far most economical investment is the special repayment or return!

But believe me, your 20,000 buffer will be gone in the end! Especially if you haven’t properly planned the garden (carport, paving, planting, etc.?).

Everything is planned. I just said that it could also be done later.

The problem is finding someone who really knows what they’re doing and doesn’t just want to make money off me.

You could also put the cart before the horse ... meaning define a specific heating technology and heating capacity first and then design the house accordingly. For example, if I want to use a 5kW air-water heat pump, I need certain U-values to achieve a correspondingly low heating load.

But ultimately, economic efficiency is decisive for me. Savings on heating technology and electricity must be in proportion to the investment in the corresponding insulation. Payback period 10 years? 15 years?

And that is exactly what every builder should probably do. Whoever plans the house and provides the thermal protection certificate should be asked how the economic efficiency of the project looks. It is important as a builder to ensure that financing, maintenance, and service life of the components are taken into account. If the builder is only interested in comfort, that is a completely different matter.

I’m curious how my architect will react when I ask him these questions and what arguments he will have. One thing is for sure: I want to see numbers. I’m not interested in empirical values; I’ve heard that way too often.

Who proceeded the same way? Who just built KFW xy because it was recommended and because it’s logical that if you insulate the house and get a better interest rate you save money. Come clean :D

I just have to think about the conversation with Viebrockhaus when I said I only want to build according to the 2014 Energy Saving Ordinance ... “We only build from Kfw55 standard ... unfortunately, we can’t do anything for you.” That probably means that everyone who builds with Viebrockhaus is building uneconomically for the next 10-15 years?

As already mentioned, it is still not possible to build economically. Nothing pays off; only costs can be reduced or deferred.
You are merely considering with which additional investment you will save more money in the long run than you have invested. However, you usually do not get anything back.
So just set a time period, for example, a measure should have "paid for itself" within a certain period (e.g. 15 years). Then you can calculate how much energy you roughly save (values like heating degree days etc. can also be found on the internet for your region) and whether it suits you or not. However, despite all the math and physics, it will only be an estimate (there is no 100% calculation; for that, you would have to know every screw). Therefore, you can also exaggerate this point; the calculations are only meaningful if there are "big" results, you cannot calculate heating costs to the exact 100€ (user behavior, weather, etc. already play too big a role!).

The heat performance of the insulation decreases inversely with thickness, i.e., it quickly no longer "makes sense" to insulate even more. To halve the heat loss through the wall (i.e., halve the losses through the wall), you have to double the insulation (double the costs), but you only save half of the consumption costs. If these are already low, the savings quickly no longer stand in relation to the costs. High KFW values or passive houses are more for the conscience, as the standard is already so high that you cannot save that much anymore.

Furthermore, I have to say that comfort should not be forgotten either. Maybe for me, a pellet heating system and omitting the [Controlled Residential Ventilation] (whose heat recovery at most reduces the loss) would have been more "economical." But I do not want to miss the comfort of the [Controlled Residential Ventilation] and also the heat pump. With these, I have practically no work/effort, but always good air and a suitable temperature without effort.

On paper, I have KFW-70, as in my opinion more would not have been worth it. However, I did not even try to reach 55. On the system side, that was already over, and on the insulation side almost achieved, but the costs for the effort (experts, etc.) would have already swallowed the KFW bonus. In actual consumption, I would have had hardly any noticeable difference.

With an average annual performance factor of 2.5, that is 140 kWh electricity consumption per year. At €0.27 / kWh, that is just under €40 savings per year. Where is the calculation error?

As has already been written here, you have calculated everything correctly. Saving through the walls is relatively difficult anyway, since only a small percentage (about 15-20%) of the heat is generally lost there. Windows and the roof would be more of an approach, but of course, it wouldn't be worthwhile either.

Personally, I still fancy a 42.5 wall, as this should provide an advantage especially in summer as heat protection (keyword phase shift). In addition, there is slightly better sound insulation and deeper window sills (not everyone’s preference). But of course, it depends on the additional cost. You should also consider that the interior dimensions of the house will then be correspondingly smaller or you have to plan somewhat larger.