Is the standard land value for a building plot in a private sale binding?

In the case of a very favorable sale, the tax office may assume a hidden gift. The exemption amount for gifts to nephews is €20,000 every 10 years. Amounts exceeding this are taxable under tax class II. A purchase price below the standard land value is initially not a problem. Do it without greed, communicate with the tax office, and pay a few euros in taxes if necessary, and enjoy the affordable property. We live in peace, use a comparatively good infrastructure, and are safe, not having to worry about going out on the street at any time of day or night. That is a privilege when you look around the world and certainly worth some tax money. Anyone with a small amount of ethics and self-respect will not sell themselves and their reflection for a few thousand euros. Of course, I know there are also plenty of people for whom money is more important than decency – I'm not naive.

It also depends on the question of what is ultimately cheaper. If you get the property extremely cheaply, it will probably remain cheaper even with the taxation of the "gift" than if you shift the purchase price in such a way that the tax office doesn't say anything. BUT in case of doubt, you would have to calculate/clarify this exactly.

Has there been any development here? I would also be interested to know what "discount" a tax office accepts.

No, not yet. Tax offices themselves provide different information. One says you are not allowed to deviate at all, another says you can deviate arbitrarily in private sales. The first contacted tax advisor believes you are hardly allowed to deviate at all. The appraisal committee (which sets the standard land values) suggests commissioning a market value appraisal, which in their opinion will not fall below the standard land value...