Construction contractor billing according to VOB - How to calculate?

Take a look on eBay, almost every builder ends up with leftover bricks. It’s probably always calculated very generously because delivering more later is extremely expensive (and it can cause other problems if a new batch is used for the last rows). Also, a good mason is expected not to use every single brick, but to throw away a few unsightly, damaged ones here and there.

Openings in the facade are not deducted. There is some minimum size at which this must be considered in quantity surveys. Otherwise, you would have to deduct every little thing.
Does it add up then?
Just ask him.

Why is this actually the case? I was also surprised that the 20 sqm window area was not deducted in my case. That’s over €3,000 that can be billed but were not actually performed. Only the quantity for the area minus the 20 sqm window area was ordered for the bricks.
"Openings under 2.5 sqm are measured overhead" was also stated in the final offer after my inquiry.

Why - no idea, it is probably stated in some set of rules. But it is also common practice among plasterers and painters, as far as I remember.

I don’t know about the plasterer and painter, but the bricklayer probably has increased effort due to openings compared to “bluntly building through” (determining exactly where the hole should go, deviation from offset-laying the stones to create straight edges, etc.). This extra effort is only compensated by the elimination of the actual opening once the opening is large enough. That’s why there is this limit (which I have no idea whether the specific size is actually the “right” one). However, I was also under the impression that this limit is at 2 sqm, not at 2.5 sqm.

In a unit price contract, it must generally be noted that in the event of a quantity deviation (more or less) of 10%, the contractor is entitled to rights. If actually less than 90% of the tendered item is required, the contractor can increase the unit price for this item.

To solve the puzzle. They had simply miscalculated.