Building in existing structures - Subsequent additional costs despite a fixed price

No one here will be able to answer that for you. All we know are individual fragments of the contract. However, a contract for work is a complex overall work. I believe the most sensible advice that has already been given to you several times here was to have it reviewed by a specialist lawyer. Not to have your general contractor then legally contacted, but purely as legal advice for you. Because what you have negotiated and signed in these contracts ultimately forms the basis of the entire contractual relationship. These "subsequent costs incurred" are not really subsequent; it is rather the question of whether this is contractually covered anywhere, and if not, what the legal situation would be.

I currently rather "fear" that with 50k you have already significantly overpaid the contractor and maybe that's why he is charging you additional costs, because he doesn't necessarily want and/or can finish your project. What you have already paid can initially be recorded as probably lost in case of separation, no one will voluntarily refund you anything and waiting years for it is also not nice...

What room for maneuver do you have? None... try to negotiate the 20k and then you have to live with it. Legally you may be right but what good is being right legally if you want to build a house... unless you have a lot of time and can do without moving in for the next 2-3 years

Based on what you have written here, in my opinion, you are in the right. I would tell the contractor exactly that. The reasoning will be provided to you by the specialist lawyer. Considering that you have already paid 50k, a legal dispute is probably not the desired solution. Agree on 50% of the additional amount. Make it clear that this is a huge concession on your part. And you assume that this way the construction will be completed on time in the mutual interest of both parties.

Do you already have the building permit?

I am shocked at how many here tend to pay/settle immediately even though I can partially understand the arguments. If you believe you are right, then refuse with reference to the contract clause, etc., and then wait. In my opinion, this puts you in a better position... So I wouldn’t just have 20k to spend because a [NA] shows up at the door...

The building permit is available.
What difference does that make for you?

Otherwise, at this point, I can only express my heartfelt thanks for your open words and honest assessments.

We are now seeking a constructive conversation. We want to avoid a legal dispute, as lawyers usually benefit from that.
We expect a significant cost contribution from the GÜ and will communicate our own considerable cost contribution as a clear gesture of goodwill on our part.

Let's see what the outcome will be, despite my bold announcements.

I am shocked by how many here tend to pay/settle immediately even though I can partly understand the arguments. If you believe you are in the right, then refuse with reference to the contract clause, etc., and then wait. In my opinion, that puts you in a better position... So I wouldn’t just have 20k lying around to spend because an NA suddenly shows up...

I would handle it that way too. I can’t just print money either.