Problem after window replacement | Measured incorrectly?

Honestly, I would just throw the windows out as they are. How can you produce such botched work? It really shouldn't and can't happen.

I would also prefer that, the only question is, who will cover the damage, and would I be in the right if I have everything redone? This would only work if all costs are covered by the window company?

I would prefer that too, the question is only who will cover the damage, and would I be in the right if I have everything redone?

The contract with the window installer needs to be clarified. With a verbal statement that the measurements will change due to planned insulation, you will have little chance of enforcing any involvement from the window installer. The window installer cannot be held liable if the conditions at the time of measurement do not correspond to the conditions at the time of installation or subsequent work on the house, unless this is explicitly stated in the contract.

But doesn't a company have a duty to inform the customer about this? What if there were not 5cm of plaster but 15cm of plaster on the sides? Then none of this would work anymore? The layperson can't be held responsible for that, can they? What are trained professionals for then?

... have you read what hampshire wrote? What have you verifiably (in writing) communicated to the window installer about the future work on the exterior wall?

That an 18cm VWS is attached to the exterior facade

But doesn't a company have an obligation to inform the customer about this? What if there were not 5cm of plaster here but 15cm of plaster on the sides?

Then it would have been sufficient to find a suitable fastening with the same dimensions. The window manufacturer has no obligation to point out to you that the window dimensions might no longer fit if you make structural changes to the house. Literally translated: If I buy a tailored suit and do not inform the tailor that I intend to significantly change my weight and want the suit to fit the changed condition, he has no obligation to inform me that the suit might no longer fit if my body shape changes.