Independent building expert - Necessary or alternatives?

Yes, that all sounds good ... but unfortunately difficult in practice

How is the expert supposed to estimate how big a defect is? Whether it can be left as is or needs to be redone? If he says redo: will you really do that even if a lawyer says that would be disproportionate?

These are the things in practice that make it difficult

A proper expert can estimate that and should also know the tolerance limits. Likewise, they should be able to roughly estimate the values behind it. We don’t need to argue over the euro. Our expert was even able to calculate for us what it would cost if we had to correct a wrongly installed underfloor heating. It should be said that we had an old-timer who had a huge wealth of experience.

So we had an expert via the Bauherren-Schutzbund. It was about 300 euros per visit, I think. We always brought him in when we needed him. So when the shell was up and the roof truss was on, when the heating and pipes were installed, for the electricity again, and sometime in between as well. There were reports with photos every time. It was important to us that someone looked at it when it came to the fundamental construction phases. For tiling we didn’t need anyone anymore. If that is botched, it’s annoying, but it doesn’t ruin the whole build.

We did exactly the same with an expert from the Bauherren-Schutzbund; costs were similar. It was extremely helpful, especially since we still lived and worked far away during the construction phase. Surprisingly and pleasingly for us: no one ever questioned anything they wrote in their reports. She also works very well with a (unfortunately necessary) lawyer – also recommended by the Bauherren-Schutzbund. You can really spend your money on more pointless things in construction. Costs for the expert approx. 3.5k; for the lawyer approx. 2k. Considering the total amount, these are really peanuts. We can honestly only recommend it to everyone. The construction companies and their subcontractors obviously have fully booked order books; they sometimes work very hastily and thus of course with mistakes. As a layperson, you would have a very hard time without support. I won’t name any names here yet, let’s see how the negotiations about “offsettable counterclaims” between our lawyer and the general contractor turn out. After that, possibly a detailed report...

We had a civil engineer from the local area; we negotiated the distance allowance. He was on the construction site countless times and showed presence. That alone is enough to prevent some construction blunders caused by carelessness. For 3000 euros, there were occasional clashes between our expert and the site manager. In summary, we were spared from botched work and have not had to report any complaints even after 2 years.

A proper expert can already assess that and should also know the tolerance limits. Likewise, they should be able to roughly estimate the values behind it. We don't need to argue about the euro.
Our expert was even able to calculate for us what it would cost if we had to correct a wrongly installed underfloor heating.
It should be said that we had an old hand who had a great wealth of experience.

As an example: your site manager did not lay bitumen or Katja Sprint or something similar on the floor of the WU basement... instead, he had an alujet foil used.

The underfloor heating and insulation are already installed. Your expert says this is wrong, everything must be removed and redone. Your site manager says he has been doing it that way for 20 years. On the internet, you find contradictory statements.

Do you now stop construction and tear everything out again, even though the site manager and craftsmen say the build-up also works as it is???

Who pays for all that then? Are you getting a lawyer now? What do you do?

To come back to what I said at the beginning; implementing the expert's recommendations nowadays is extremely difficult. Unless you have someone who really stands by your side and intervenes at every little step. And it is also difficult to estimate what actually works in practice and what doesn’t. In my opinion, unfortunately, the only help is to inform yourself beforehand...

As an example: your site manager did not lay bitumen on the floor in the WU basement or Katja Sprint or similar .... instead he had an Alujet foil used

I think this is a bad example. In the past, this was viewed more critically, today we know more. A vapor barrier is sufficient with properly executed WU.

But then I find this to be a good example. You seem to know this now and would therefore definitely not have everything torn out. The expert said everything has to be removed.

In this example, I am the layperson and don’t know much about it and would do what the expert says.

With regard to your statements that we now know more, this would be completely excessive. The construction manager knows this and shakes his head, as does the craftsman. Unfortunately, as a layperson, I don’t know and cause costs for which I don’t know who will bear them.

From the other side, the expert is right, these are not the recognized rules of technology.

Which goes back to my initial post: the difficult thing about the whole matter is the decision of what is proportionate and what is disproportionate.