Final invoice with new (traceable) items

You have general costs as a construction company per construction site.

You mean the site overhead costs (BGK). But he definitely has an item "setting up the construction site". Those costs are included there. Also, it was always only about a reduced quantity, not about 40%.

The issue for the OP is the question of the additional calculation of the "Gratausbildung als Zulage zur Dachfläche" and not about the 9% surcharge....

From the feeling, I would say that he should have informed you beforehand.

The roofer carried out the ridge formation and points out that the execution is necessary for the roof covering and therefore a customary local fee may be charged - without prior notification.

How many euros is it about?

If I had a craftsman who caused delays that were not comprehensible, deceived me, and came up with an item on the final invoice that had not been communicated beforehand against better knowledge and was therefore surprising to me, the craftsman would have to sue me for this item. He would never, ever get away with it. Consumer protection, Building Code - nothing to do with VOB. If the cooperation had been consistently pleasant, I would of course pay it, even if I could avoid it.

If I had a craftsman who causes delays that are not comprehensible, deceives me, and comes up with a position in the final invoice that was not communicated beforehand against better knowledge and therefore surprises me, the craftsman would have to sue me for this position. He would never get away with it. Consumer protection, Building Code - nothing VOB. If the cooperation had been consistently pleasant, I would of course pay that even if I could avoid it.

That's exactly how I see it too. On the one hand, I want to pay for the consumed service fairly, but somewhere it is also too much. In the bill of quantities, there were no items like site setup, etc., but quite specifically the individual executions. We found the 9% surcharge for the pure roof fair. This was also indicated early on, and our site manager said it would be fine. The roofing contractor's justification was that for 90 m² you get a different price for the tiles than for 120 m², etc... Well, I don't quite believe that now, but in general it is true that the bigger the roof, usually you get more percentage discount because, as mentioned, costs for invoicing, travel, etc. are distributed over more square meters. Keyword mixed calculation. In the end, the roof was also cheaper than the offer. The ridge training was estimated at €59.65 per meter. In total, this involves €1,558.06 net.

Primarily, the fault lies with the client/site manager due to the error in the tender. The roofer then calculated and offered the required service. During execution, it would have been better for all parties involved to declare the service as additional costs beforehand. Since the ridge was necessary to fulfill the service, I see a claim by the roofer as justified. The amount is now the only disputed issue. I cannot assess how expensive the ridge is. Is the roofer willing to negotiate??