Damages to plastic windows

How could something like this have been prevented.

Basically not at all, if you have idiots on the construction site. The frames themselves already have a protective film on them. You can also buy a protective film for the glass. But if someone steps on the frame or bangs heavy equipment against the windows, you are powerless. BUT: Such behavior on the construction site is not the rule.

We took the trades that were recommended to us by our architect. He has been working with them for a long time. I can't say that we always chose the cheapest ones. Honestly, that's not really the case.

The shell construction, drywall, interior and exterior plastering are handled by one company with subcontractors. This is also the one that caused most of the damage.

The issue with the bitumen and the roofer still needs to be clarified. Our architect is taking care of it.

If someone steps on the frame

Can you provide some kind of step protection there? I imagine something like a cable protector...

banging against the windows with heavy equipment

In that case, you are indeed powerless, although probably no scratches would remain but rather something really gets broken (fracture), where then no one could argue that it was like that before. And for everything that scratches without bubble wrap, wouldn't such a thing prevent some of it?

With the shell construction company, as I just wrote, you can clearly tell that they - to put it bluntly - don't give a damn whether there is any damage somewhere. It's just a construction site - that's how it is, they say. We're supposedly so picky and just making a fuss. They also completely trashed the outside area with food scraps and other stuff. I'll be glad when the drywall work is done and I don't have to see any of them there anymore. You can really only say they were real slobs.

Unfortunately, this aligns with my experience with most (but not all!) craftsmen. Drunk on the construction site, leaving piles of garbage, working only when instructed and definitely never thinking for themselves. In my experience, craftsmen earn too much money for the poor quality work they often deliver.

I'm glad I don't have to see any of them anymore and now do almost everything myself. Cheaper and with much better quality. And that as a "trained engineer", who is often laughed at by craftsmen for having two left hands...

Even here, pictures say more than a thousand words.