Window offers like day and night

But it will.

Old pessimist.
Currently, the plan is: The windows will receive reveals insulation of about 4 cm and will be made correspondingly smaller.

Following variants:
1. We don't need the reveals insulation at all for the Energy Saving Ordinance - in my assessment, not unlikely. Then we have it for nothing.
2. The reveals insulation is too weak. Here, improvements could be made with material selection or possibly compensated elsewhere by the 40% rule.
3. The windows are damaged when the ceilings are removed. Then the money is lost - but it would be the same with the tax office.
4. The windows are visually unsuitable - a fate that pains me greatly but could no longer be changed.

Then I will paint white: 1. / 2.: insulation will improve the values even if it misses the target - i.e. instead of the optimum, it will probably at least achieve an acceptable approximation; 4.: undivided and without struts, I see this risk as low, paint can be changed cheaply.

Regarding 1/2) No matter what you do, jamb insulation will never be mandatory. If the façade is considered as a separate measure, it must overall meet the requirements of the Energy Saving Ordinance and not per brick. And within the framework of the 140% rule, even the entire façade may fall below the requirements of the Energy Saving Ordinance, as long as compensation is made elsewhere.
Regarding 3) What is the deal with the tax office?

Edit: To my knowledge, the usual programs do not even offer the possibility to calculate the influence of jamb insulation on the U-value of the wall or the window. It is solely about the issue of thermal bridges, and these only play a role if they are consistently taken into account for each component and then a detailed proof is provided. Those who, like most "energy consultants" for old buildings, dismiss it half-heartedly and simply use the flat 0.1 value, end up getting nothing out of it.

So far, I have not read, seen, let alone understood anywhere who demands a cavity wall insulation, how, and why. However, both the consultant and the window manufacturer independently insisted on the 4cm insulation, and due to lack of time and better knowledge, we more or less complied with them.

Today, two energy consultants called who would take a look at the matter. I think we will choose one of them and get further advice. But first, we will check the static structure.

I don’t have a complete overview of the tax office either. Something like this: until March, provisions for deferred maintenance from the previous year can still be deducted from taxable profits.

Where did you source your windows from? Are there any recommended suppliers for Polish windows? I would also order them directly if possible or necessary, but I lack reliable addresses. Apart from a large sliding door, we will handle the installation ourselves.

The offer is from a window installer from Dresden. I'm afraid he won't drive to BW for an order now. I (still) don't know the Polish company. Maybe I'll find out... but not today.