Window offers like day and night

We also found this page. There is really a lot explained about the [Energieeinsparverordnung]. Thank you very much! The difficult part of the project will probably be gathering various regulations for the repurposing in the end – not just the [Energieeinsparverordnung].

For the repurposing, I would look for an experienced local architect. It is often less about regulations and more about political/administrative goodwill. The better this is prepared and justified, and if the architect ideally has a good connection to the person responsible, the greater your chances of getting it through without major pitfalls.

From personal experience:

Our landscape gardener owns an inherited farmstead in the rural area and had started to "illegally" set up 4 apartments there. When this came to light during the construction work, in addition to the corresponding payments, he was required to dismantle the apartments.
By chance, at a birthday party that weekend, he spoke to a well-established architect about it, who had the brilliant idea to offer the apartments to the city for housing asylum seekers and tie this to the condition of granting retroactive approval.
It worked.

Well, we thought we had him. But so far, his plans are far above the [Energieeinsparverordnung], which we do not understand. I have to talk to him again.

But so far he is planning well above the Energy Saving Ordinance, which we do not understand

As already wrote, the requirement of the Energy Saving Ordinance on the primary energy demand (reference building 2014 without tightening +40%) does not necessarily have to be met for existing buildings. Only each individual component that is changed must be designed accordingly OR the overall value must be achieved.

after all 60 cm thick walls

60 cm with clinker facade and the windows are hinged on the inside. Have you checked whether the facade is double-layered with an air gap? It really smells like that to me. Even though blown-in insulation is sometimes controversially discussed, you could gain a lot here: inexpensive, preservation of the appearance, no additional reveal insulation necessary.

60 cm with clinker facade and the windows are hinged on the inside.

No, not at all. This is asked all the time. No, the windows are all bone dry. We have never had problems with mold or moisture on the windows.

Have you checked if the facade is double-layered with an air gap? It literally smells like that to me.

The wall is almost certainly solid. We have multiple holes in the wall where you can see it, and various spots have been drilled through – there was never any cavity to be found. However, more drillings will be done soon, and the quality of the bricks will also be determined for the statics.

No, not at all. This is asked all the time. No, the windows are all bone dry. We have never had any problems with mold or moisture on the windows.

LOL

Sorry, but too cute

What Till means are the "window stops" that I had already suspected. The rough openings of the windows in the masonry are slightly smaller on the outside than on the inside; so the window is placed in front of a kind of frame from the inside - this frame is called the "stop."

The stop logically covers the window profile from the outside, so the space for insulation is reduced.