Additional cost breakdown Outrageous!

that's correct. If the contractor has only verbally announced that there will be additional costs, that is already sufficient. However, it is clearly stated that the announcement MUST be made

Especially since VOB is rarely (effectively) agreed upon with private builders

This concerns a modified service according to § 2 (5) VOB/B "geänderte Leistung". The AN is entitled to a different price if this was agreed upon beforehand. Now the provision of the service corresponded to the presumed will of the clients, so the claim still exists. Therefore, a claim of €49.90 for sampled parquet minus €30.00 for already included tiles = €19.90 times the area could exist. I would assume the working time to be the same. I would pay the money without acknowledging any legal obligation; the rest would have to be litigated by the AN. If necessary, you can still pay it during the court hearing. However, many do not go this route.

If necessary, you can still pay it at the court hearing.

plus how much fees?

Well, none, the court demands the other party. He will not sue, the litigation risk is too high. He will always seek an amicable settlement.

Huh? You write, if necessary pay at the appointment. That only makes sense if the other party has a claim, meaning you lose. And a settlement almost always involves splitting the costs.

For the disputed amount, the fees are higher than the amount in dispute.