Additional cost breakdown Outrageous!

That is only the material cost for the flooring, what about the labor?
Laying a room is also far less labor-intensive than covering a staircase.
It is also a different trade, own calculation, etc., you won't get the exact same labor price the tiler previously set.

Did the BU really never point out the additional costs?
It is obligated to do so.
I had a similar case...

Did the BU really never point out the additional costs?
It is actually obliged to do so.
I had a similar case...

Yes? Always? Or only in certain constructs?

Always?

Certainly not in unit price contracts. And in the majority of cases, not with quantity increases either.

Here is an excerpt from the VOB:

6.
If a service not stipulated in the contract is requested, the contractor is entitled to special remuneration. However, he must notify the client of the claim before commencing the execution of the service.

The remuneration is determined based on the principles of price determination for the contractual service and the special costs of the requested service. It should be agreed upon, if possible, before the start of execution.

7.
If a lump sum is agreed upon as remuneration for the service, the remuneration remains unchanged. However, if the executed service deviates so significantly from the contractually agreed service that adherence to the lump sum is unreasonable (§ 313 Baugesetzbuch), an adjustment considering the additional or reduced costs is to be granted upon request. The basis for the assessment of the adjustment is the principles of price determination.

The provisions of paragraphs 4, 5, and 6 also apply when a lump sum is agreed upon.

If nothing else is agreed, numbers 1 and 2 also apply to lump sums agreed for parts of the service; paragraph 3 number 4 remains unaffected.

VOB

Have you ever double-checked how much various supreme court rulings on the VOB, especially §2, para. 6 VOB/B, leave standing in practice?
Spoiler: Not much.