4 years after construction, the office also demands land acquisition tax on the house

We then insisted on paying real estate transfer tax for both.

Because you didn't understand the context.

Smarty pants. Neither did I, but then the seller wouldn't have engaged with the OP as the buyer.

What was conducted here was a land purchase contract between the seller and the OP. This is primarily a contract that requires notarization. To fix a side agreement "extrajudicially," I think not only I consider nonsense. The general contractor wanted no security here at all, he was not involved in this clause at all. It was the seller who wanted to avoid bringing a loner on board whose horizon ends at the shared border like that of Goalkeeper's Mittelhäusler and Neo-Mittelhäusler. He merely applied a not necessary and thus inappropriate means to the purpose for this otherwise legitimate interest. What would have sufficed, I have already stated: that the OP submit to a joint planning. With that – but probably not in bad faith – he caused the OP financial damage, yes. Wasted money, out of stupidity, but this wound also burns without additional salt being sprinkled in.

... and in southern Germany, official notaries - sometimes even with areas of jurisdiction - are common.

I have never met a notary who was not also a lawyer.

No, that was abolished as of January 1, 2018.

If I needed information about real estate transactions, I often went to the notary's office hours. Better than the lawyer and simply costs nothing.