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

And I agree with the initial suspicion that the notary may have provided inadequate information.

Or the original poster was already so stubbornly in denial at that time and did not want to see a linked transaction, since he did not yet have a contract...

A notary is not allowed to provide any tax advice. He is not authorized to do so.

If you ask him about it, he might possibly point it out in a non-binding manner, but not as tax advice. Advice, here communicated for what feels like the hundredth time, is generally free!!!

In this case, one could have informed oneself before the notary's contract draft whether the aforementioned purchase agreement triggers a tax obligation or not.

When I needed information about real estate transactions, I often went to the notary’s consultation hours. Better than going to a lawyer and it costs nothing.

A little tip. Most notaries also hold special consultation hours on the topic of inheritance and wills. Free of charge!

A notary is not allowed to provide any tax advice. He is not authorized to do so.

Complying with his duty to inform about legal consequences is not advice. Advice would be if he gave design suggestions here on how to avoid a mentioned disadvantage. Again, it is advantageous to choose a notary who is also a lawyer: because as a notary he only has to certify, not previously have acted as a lawyer in the same matter ;-)

Short info for you. It is rare for a notary to also be a lawyer.
However, if that is the case, the following applies here:

Notaries - unlike lawyers - are not representatives of a party, but completely independent and impartial guardians of all parties involved. In their independence, notaries resemble judges.

It is not the task of notaries to authoritatively decide disputes and other matters. In this respect, they differ from judges; rather, they offer advice and participation to the parties involved. The parties are free to accept or reject the advice.

To avoid even the slightest doubt about their impartiality, notaries may no longer act as notaries in a matter in which they have already acted outside their official function. Thus, a lawyer-notary may not notarize a matter in which they (or a person participating in a firm) have already acted as a lawyer. Conversely, the lawyer-notary may not act as a lawyer if they have already acted as a notary in the same matter. Due to the special importance of notarial impartiality, the term "same matter" is to be interpreted broadly.

So, no legal advice and notarization for the same client.

I have never encountered a notary who is not also a lawyer. In fact, 2/3 of notaries obtain their office through a specific training path (which does not exclude practicing as a lawyer) after the 1st/2nd state examinations as lawyers, and 1/3 are appointed as practicing lawyers.

Ultimately, you are discussing a duty to pick up or deliver here. Those who are (pre-)aware recognize the tied contract. Even the forum members managed this without having the text, purely from the designation of the construction obligation with the one general contractor.

It is not the case that the notary reads every sentence aloud and works through all conceivable legal consequences. They don’t have to. As a client, you can however demand this or ask explicit questions, yes, but regarding poor advice ... I do not see that. We also do not have the wording of the construction obligation with the general contractor here, so no one can say whether a layperson without prior knowledge should not have noticed that. You can phrase things very explicitly in contracts or not. We do not know here. But the fact is, the OP overlooked, did not understand, or repressed it. Anyone who now believes to conjure money out of "poor advice" is head in the clouds.

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

Notaries are fully qualified lawyers and could also be judges, attorneys, or public prosecutors. But they are by no means only lawyer-notaries. Personally, I know far more notaries without a law firm, and in southern Germany, official notaries – sometimes even with jurisdictional districts – are common.

It is not the case that the notary reads aloud every sentence and goes through every conceivable legal consequence. They don’t have to, either.

Correct. A notary does not need to conduct a beginner’s law seminar. But the note that the tax office becomes aware and also taxes property acquisition related to construction accordingly, in my opinion, should be part of an appropriate explanation. Notaries tend to preach such extensive liturgies; you don’t need to omit the amen, especially not ;-)