ᐅ Replacing windows / Is the quote fixed or negotiable?

Then you’ve already answered your own question.
I’m always surprised when people ask questions and then get snappy at someone who knows better from experience, accusing them of not knowing anything...
My friend, I have worked professionally with installation companies from Eastern Europe for more than 12 years. The experiences range from "I have nothing to complain about anymore" to "we’re hiring a German company because nothing fits here at all..."

Buy the windows in Poland. Usually, it will go well.

If it doesn’t go well... then you have a problem.

That’s all I said, no more and no less.

Whether you think that is worth 1,500 EUR (about 1,600 USD) less in the end, is a decision only you can make.

Sorry, but I’m not being rude just because I’m correcting you. I don’t really understand what your experience with "installation companies from Poland" and "I should buy in Poland" has to do with windows from an established German window manufacturer, but alright. You seem to be getting a bit confused. Especially since German companies now also operate locations in Poland. However, this shouldn’t turn into a Germany vs. Poland discussion here.
I just wanted to clarify that the manufacturer is not the point of contact for warranty claims.

rennschnecke schrieb:

How do you come to the assumption that the installation is 100% flawed?
Yes, there is a special soundproof installation method for acoustic glazing.
Why would it not matter which glass you choose when paying an extra charge? I do think soundproof glass makes a difference.

First of all, I overlooked the size. Sorry.
How do I come to that conclusion? Because at the price you pay for the windows, a RAL-compliant installation including materials is not financially feasible. The windows are already very cheap. And then there is only a small extra charge for soundproof glass—what benefits does that really offer? You don’t get much value for the additional cost.
That’s why I said you will have to make compromises during installation, so the soundproofing will not do much good, and the soundproof glass will only have a very minimal effect.

rennschnecke schrieb:

The energy certificate for the house shows a value of about 80 kWh/(m²·a).
We only heat the living and dining rooms as well as the kitchen with the wood stove; the bathroom is heated with the central heating. I would have to check, but our gas consumption is extremely low for the floor area.

So heating costs are not that low after all—wood. What about the other rooms? What the energy certificate states is irrelevant. The actual consumption matters. And for a 25-year-old building with drafty windows, it is physically impossible to maintain high indoor temperatures with little energy input from the heating. The insulation is insufficient and there are drafts.
If I heated my entire house with just a wood stove and didn’t use any gas at all, would you then say you live in a passive house and don’t need heating? I find statements like that—which come up quite often—such as "we don’t heat much" are later qualified with phrases like "but upstairs we don’t turn on the heating because the kids don’t live there anymore, and downstairs the wood stove is on in the living room, and 16°C (61°F) is sufficient in the kitchen and bedroom."
So my assumption that you do NOT actually heat very little has proven to be correct. It may be that you heat cheaply because you get wood for free. But that is a different matter.

rennschnecke schrieb:

He was also the only one who mentioned a "soundproof installation" to us.

And what did he tell you about the ventilation concept? The likelihood of mold problems in your home is very high with drafty old windows and (even if installed correctly) then hermetically sealed windows. So far, you have basically had a ventilation system (or something like windows constantly being left slightly open) that ensured some air exchange after replacement. That must no longer be the case in the future.

rennschnecke schrieb:

I don’t really know what your experience is with "assembly companies from Poland"

I can at least tell you that Polish craftsmen are among the most sought-after because the quality of their work is excellent (they have good training). If you are building a new home, it is quite common for the German main contractor to source the windows and installers from Poland as well. Nothing unusual about that.
Of course, in that case, you still have a German contract partner.

nordanney schrieb:

How do I come to that conclusion? Because at that price for the windows, a RAL-compliant installation including materials isn’t financially feasible. The windows are already very cheap. And then there’s only a small additional charge for acoustic insulation glass – but what benefits does that really offer? You don’t get much value for the extra cost.
That’s why I said you’ll have to make compromises on the installation, which means the acoustic insulation won’t be very effective and the soundproof glass will only help minimally.

We have now received a second offer (Schüco Living 82MD), also with soundproof glass, which is roughly in the same price range. So somehow I can’t really understand the statement that we have to make compromises.
The benefits of the soundproof glass? Well, it reduces some of the noise from the adjacent street.
As I said, I still don’t understand how you can accuse the company beforehand of doing a sloppy installation.
Should I now look for a company that charges 20,000 euros because that way it’s guaranteed to be installed professionally?

nordanney schrieb:

And what did they tell you about the ventilation concept? The likelihood that you’ll have mold problems with your drafty old windows and (if properly installed) then hermetically tight new windows is very high. So far, you basically had a ventilation system (or something like constantly tilted windows) that prevented that issue. That must not happen in the future.

We have not discussed a ventilation concept with any company, from the window manufacturer to the joiner, etc.
However, we never had mold problems in previous apartments. I believe that proper ventilation will prevent it. We have never had mold issues due to sensible airing in any apartment so far.

nordanney schrieb:

Then my assumption that you do NOT heat little was correct. It may be that you heat cheaply because you get wood for free. But that’s a different matter.

We don’t get wood for free.
Gas consumption for 190 sqm (2045 sq ft) is about 17,000 kWh, which also covers hot water.
Our wood consumption is very low as well.
But basically that doesn’t matter much, since it was more about assessing the offer.

MachsSelbst schrieb:

Buy the windows in Poland. Usually, it will work out well.

If it doesn’t, then you have a problem.

That’s right.

rennschnecke schrieb:

In our previous apartments, we never had any mold problems. I think proper ventilation prevents that. So far, we haven’t had mold issues in any home thanks to adequate airing.

You actually didn’t have to ventilate in that house. Mold problems after window replacement affect many property owners, and even more tenants and landlords (and thus experts and courts) on a daily basis. Believe me. Keep these issues in mind.
And as I said, a ventilation plan is not optional, but mandatory. If you want a proper installation, you also want a prescribed plan.

rennschnecke schrieb:

The gas consumption for 190sqm (2050 sq ft) is about 17,000 kWh, including hot water.
Our wood consumption is also very low.

LOL. That is not low. That amounts to roughly €2,000 in heating costs. Physics—as stated. You can’t heat cheaply with that.
If I convert my house (which is somewhat smaller) to 190sqm (2050 sq ft), my heating costs including hot water are around €500–550 (22°C (72°F) room temperature). That’s what I mean by subjective.