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

My tax advisor described to me non-bindingly (thickly underlined), since contracts are not available, that even if we agree on a general contractor in advance in order to possibly secure funding and sign regarding the house construction, there is no uniform object of acquisition and no objectively factual connection, since we are still free in the "if" and "how" towards the seller. In their opinion, there is no legal or other connection between the purchase contract and the general contractor… I still feel a bit uneasy. The notarization of the property is dragging on forever, as there is no approval from the municipal utilities and we would very much like to obtain the KFW funding commitment already (and secure the current prices).

My tax advisor described to me non-bindingly (heavily underlined), since contracts are not available, that even if we agree on a general contractor in advance to possibly secure funding and sign in regard to the house construction, there is no uniform object of acquisition and no objectively factual connection, as we are still free in the "whether" and "how" towards the seller.
In their opinion, there is no legal or other connection between the purchase contract and the general contractor...
I still feel a bit uneasy. The notarization of the plot is dragging on forever because there is no approval from the municipal utilities and we would very much like to already obtain the KFW funding commitment (and secure the current prices).

Why do you feel uneasy? Even the lawyer describes exactly how anyone else would. It is only a problem if, when purchasing the plot, you are not free to choose the construction company. That has always been the case and with that you would win EVERY lawsuit against a tax office.

I also have a question regarding the draft purchase contract; I had to send the property seller a preliminary plan of the house. The draft purchase contract now states the following: "The buyer undertakes to submit a building application regarding the purchase object within twelve weeks from today and to develop it within two years from today with a residential building according to the KfW 40 plus standard and in accordance with the building and planning law as well as the concept documents attached to this deed as Annex 1. The seller points out that the city of Castrop-Rauxel has issued the development plan No. 245H for the planning area, which is attached to the reference deed as Annex 3. Reference is made to the reference deed. The requirements of the development plan must be strictly observed by the buyer when planning his residential building. The buyer is aware that the buildability of the purchase object with the specific residential development planned by him is his responsibility."

The plans of my construction project from the construction company are attached as an annex, but the contract has not yet been signed.

Could this possibly be a tied contract and therefore subject to property transfer tax on the house construction as well? I have chosen the company myself.

The following is stated in the draft purchase agreement: "The buyer commits to submitting a building application regarding the purchase object within twelve weeks from today and to develop it within two years from today with a residential building according to the KfW 40 plus standard and in accordance with building and planning law as well as the concept documents attached to this deed as Annex 1.

"Purchase object" is the plot of land. It may be that the seller passes on a building obligation to you - but when you submit the application is none of their business. Show that guy the same and keep looking. Some people have really soaked too long in hot water.

Is this possibly a tie-in contract and does it then also incur real estate transfer tax on the house construction? I have chosen the company myself.

No, I cannot see any reason for that in your case.

So I sent the land seller an offer and plans of the house we want to build before the draft purchase agreement, and as already mentioned, he included this planning as an attachment to the draft purchase agreement. So is a linked transaction then excluded?

So I sent the land seller an offer and plans of the house we want to build before the draft purchase agreement.

That would never occur to me in my wildest dreams.

and as I already said, he included this planning as an attachment in the draft purchase agreement. So is a tied deal then excluded?

The seller of the land does not broker the construction contract for you, does not offer you the contractor, and in my opinion, the completely exaggerated conditions are fulfilled even without committing yourself to contracting this contractor. I do not see a tied deal in the fiscal sense. As a "is it so that" question, the tax office must answer you definitively, they are only allowed to advise, not just give non-binding advice. But as I said, just let the nutcase go his own way.