cost of walkable flat roof

Your prices are without own contribution but execution by a professional company. Correct?

Yes, and as I said, to be taken with caution, since we redesigned the house and especially the roof was changed as a result. What would be possible or sensible as own contribution on the flat roof? I actually don't want to mess around there, not that in the end it is said that I am to blame for the water ingress...

First of all, you should clarify whether the project is even eligible for approval. Normally rather not.

Why should a walkable flat roof not be eligible for approval?