Building permit from 1931 not adhered to

I understand it as that there is an extension in reality for which there is no permit.

And not the other way around.

Oops. Hopefully no one from your building authority is reading this or hopefully you haven't demolished anything yet.

It depends on what was remodeled. Structural interventions, even inside the house, require approval.

Please simply read my description carefully, then there will be no misunderstandings. I want to apologize to Escroda; because the quote about the demolition came from ypg. I have not written anything about demolition or tearing down. Once again for clarification: Back in 1934, a "normal" single-family house was approved by the building authority. However, the construction company also enlarged the house with a 3 m extension made of brick masonry. This extension is not present on the "old" drawing. It was probably intended to serve as a stable or hayloft. In 1978, a bathroom, toilet, and another room were installed there (without approval).

Please just read my description carefully, then there will be no misunderstandings.
I want to apologize to Escroda; because the quote about the demolition came from ypg.
I never wrote anything about a demolition or teardown.
Once again, for clarification: At that time in 1934, a "normal" single-family house was approved by the building authority. However, the executing construction company also enlarged the house by a 3 m extension made of brick masonry. This extension is not present on the "old" drawing. It was probably intended to serve as a stable or hayloft. In 1978, a bathroom, toilet, and another room were installed there (unauthorized).

Yes, you are the owner or do you want to sue the construction company from 1978? You can forget it. Long statute of limitations. It is, by the way, a misconception that after years you are off the hook. My uncle got reported because of an outside staircase from 1983. Now he has to produce massive paperwork for the office. Invoices for the structural engineer etc. are already at 5,000€. You will never get legal certainty here either!!!

I would like to apologize to Escroda

Accepted.

Please simply read my description carefully, then there will be no misunderstandings.

I have read it again carefully and unfortunately have to tell you that your wording does allow ypg's interpretation.
Your reaction

I did not say anything about a cancellation

would also have been acceptable if it had not been the only one and if the two exclamation marks at the end were not there. After all, several users had already responded to you, to which I would have expected some reaction. Either by pressing "Like" to express that the post helped you, or by stating that you disagree, or by asking questions if someone writes something unclear.
You have now clearly clarified the misunderstanding, but not whether the previous posts helped you and whether further assistance is needed. In particular, your expectation regarding point 2

2. The house has stood in its current form for 87 years. Thanks to the fieldstone foundation, there is no moisture and no cracks in or on the house.

is unclear to me.

Yes, you are the owner or do you want to blame the construction company from 1978?

He doesn’t want that at all.

By the way, it is a misconception that after years you are in the clear.

He doesn’t believe that either. Otherwise, he probably wouldn’t be asking here.
He just wants to do everything correctly. And before investing a lot of money in renovation, I find the question of how to best proceed in the case of a partially unauthorized existing property absolutely appropriate here. It should be clear to everyone that there can be no general and certainly no binding answer for an individual case, but the opinions of others can be helpful for one’s own decision-making process regarding the further course of action.

You will never get legal certainty here either!!!

Yes, you will. With a comprehensive building application. Whether legal certainty is really necessary is ultimately a matter for the decision-making process.

P.S.: please don’t scream (!!!), calling (!) is enough.

I read them again carefully and unfortunately have to tell you that your wording certainly allows ypg's interpretation. Your reaction would have been fine too if it hadn't been the only one and if there weren't the two exclamation marks at the end.

Oh, it would have been enough if you, , wrote something like "No, there was no discontinuation. It is the other way around. Please read again, ..." instead of angrily (?) just attacking misinterpreted words here. I certainly did not reply to you because I am unfriendly. Sender / receiver, I just say ;) But don’t worry, I’m out of here.