Facade according to DIN18202 flawless

How do you come to the conclusion that someone has to inspect the facade? If your general contractor sends you the building authority approval by mail, the matter is settled for him.

I am not familiar with the subject, but I also think that I cannot avoid an expert, even if it costs over 3000. I also find it very unfortunate that our developer takes the side of the subcontractor and refers to DIN18202, although the defect is visible to a blind person and also for a considerable amount of time during the day when the sun is shining.

I also find it very unfortunate that our builder takes the side of the subcontractor

Why not, after all, that is his side. As far as the term "builder" is correctly used here, ultimately he is the one who sold something defective. That is also important for the question of who is actually the addressee of your claims!

I am not familiar with the subject but I also think that I will not get around to hiring an expert,

And I think that first a lawyer is needed who is familiar with the subject. If he does his job well, the opponent will probably gladly forego the involvement of an expert – after all, in the end the one who loses pays for it.

With a specialist lawyer for consumer protection, who otherwise complains about electric toothbrushes and apparently tries ridiculous strategies here, you please the opponent :-(

I have no choice but to consult an expert, even if it costs over 3000.

How do you come up with this number? Conservatively estimated: 1 hour travel 1 hour inspection 1 hour return 1 hour writing Total 4 hours, times hourly rate (1xx Euros?) roughly 500 Euros. And in my opinion, that would already be expensive.

I also find it very unfortunate that our developer takes the side of the subcontractor

Who stands on whose side does not matter. There are only two sides. You and your contractual partner, which is usually only one other (legal) person.

Preliminary message from an expert:

Optical defects on facades are to be assessed from a usual viewing distance and under diffuse light or overcast sky. Unevenness on ETICS is to be assessed according to DIN 18202. Unless you have agreed on special requirements for optical quality, it is difficult to prove optical defects.

Sorry but to me that really sounds like we have no chance. -.-

As long as no special requirements regarding the visual quality have been agreed, it is difficult to prove visual defects.

The "pattern" of the insulation boards is clearly visible under the plaster. The house looks as if someone simply pulled a fitted sheet over the facade instead of plastering it. If I otherwise wanted to buy your house, I would offer you significantly less for it, simply because the facade looks sloppy. So, visibly less money with the naked eye, due to a visible cosmetic defect with the naked eye. That is, from my point of view, the value equivalent of the defect.

Preliminary message from an expert:

Optical defects on facades are to be assessed from a customary distance and under diffuse light or overcast sky. Unevenness on ETICS is to be assessed according to DIN 18202. Unless special requirements regarding optical quality have been agreed upon, it is difficult to prove optical defects.

Sorry, but to me it really sounds as if we have no chance. -.-

How does it look under diffuse light?