Flat roof leaking, repair or build a roof structure over it?

It's a matter of mindset. You have a flat roof, it worked and works even better nowadays.
What rational reason is there to spend multiple times the amount just to get rid of the flat roof?

That is probably a reasonable basic thought.

What moved me to consider an alternative was to bring the water drainage to the outside; it is simply unpleasant on vacation trips to have to fear possible water ingress in the basement, as has already happened. Admittedly, this can be solved with regular maintenance.

Besides that, my confidence in the flat roof has already been somewhat shaken; one can only hope that the previously promised trouble-free state is now being achieved.

Another concern is the currently existing wooden formwork, or to what extent the wooden beams tend to fatigue after 55 years, which would then entail renovation again.

The appearance also worries me a bit, but what matters is what lets me sleep better.

If you don't care about that, then put an industrial roof on it. I consider trapezoidal sheets to be an inexpensive solution. If you choose a high profile, you need a minimalist substructure.

Is the flat roof well insulated?

I fear it is not only about what I would like to have, presumably an application plan (Tekturplanung) will be required, meaning the city building authority will specify the execution, or...

The carpenter said that the city building authority will probably not approve a simpler roof structure because of the noise development during rain...

Insulation: The roof is old, built 55 years ago, to my understanding: wooden boarding, air 5 cm, rock wool, wood 5 cm, plaster 5 mm

Status: The roofer has submitted an offer for a comprehensive renovation, which would include, among other things, replacing the insulation material and raising the roof edge slightly to comply with regulations: 60,000€ :oops:

Just to be safe, I got a second offer: €95,000 :rolleyes:

I'm overwhelmed by comparing the offers, I would like to post the offers in the forum without company names being visible, is there any objection to that?

Can't you go to the appropriate authority with such an offer and have it checked to see if it's not usury? It seems really high to me, but I don't know much about it. However, based purely on material costs and hourly wages, I don't believe such a sum is justified! What else was priced in there? Crane, barricading, disposal, etc.?