Tax law advice land registry entry

Then, in the hypothetical example, the woman must sell the property to him below its real value so that no gift tax is incurred?

Yes and no, the value of the property for tax purposes is still based on the standard land value. If the sale price is below that, it is a mixed gift, which is split into a non-remunerated and a remunerated part. Gift tax is still incurred on the non-remunerated part, provided that the assumed debts do not offset it. Not to forget in the hypothetical sale: this might also have income tax implications for the girlfriend.

So now income tax? There was no mention of that before in your "consultation", that one basically only needs to pay attention to gift tax and real estate transfer tax here! I can already see that you now agree that one can make quite a few mistakes in structuring this.

Another hypothetical example: The woman’s property has a tax value of 400, she bought it for 600 two years ago. There is still a 500 loan outstanding. She sells half to the man for 300, which the man transfers to her. Could a grumpy tax officer issue a tax assessment for 100 gift tax from the man to the woman since the man paid 100 more than the tax value?

A house worth 400 is to be built on it. Assuming the man cannot simply transfer her 300. The man and woman convert the loan into a joint loan with joint and several liability and increase the land charge to 900. How is the gift tax calculated for the property share now?

Or do they necessarily have to take out two separate loans secured by the same land charge? How can a house construction be properly financed from two loans with corresponding withdrawals without gifts?

So now income tax? There was no mention of that before in your "consultation", that you basically only have to pay attention to gift tax and real estate transfer tax here! I can see you now agree that one can make quite a few mistakes in the structuring here.

Of course, income tax was not mentioned before because the OP did not ask about it. Apart from that, you still do not need a tax advisor for this; a notary knows all this as well. And last but not least, this is not a consultation but simply the statement that the constellation is not so complicated tax-wise that one necessarily needs a tax advisor for it. A notary suffices perfectly, as said.
And no one ever claimed that it works completely without advice. But I would focus attention on questions that a tax advisor cannot answer but a notary can.

Another fictitious example: The woman's plot has a tax value of 400, she bought it for 600 two years ago. There is still a 500 loan outstanding. She sells half to the man for 300, which the man transfers to her. Could a bad-tempered tax officer issue a tax assessment of 100 gift from the man to the woman, since the man paid 100 more than the tax value?

What that has to do with the OP’s question I also no longer understand for sure. What exactly do you want?!
There is no "tax value" as you call it. No, the (possible) speculation gain would be calculated, which is based on the acquisition costs and not the standard land value. That would also be fair; why should he pay less than half? "Tax value," as you call it, and actual value are not identical.

If you want to carry out a sham transaction to avoid gift tax—that would be tax evasion. You should think carefully whether marriage would not be the better alternative.

A house worth 400 is to be built on it. Man and woman convert the loan into a joint loan with joint and several liability and increase the mortgage to 900. How is the gift tax calculated for the plot share now?

Or do they necessarily have to take out two separate loans secured by the same mortgage? How can a house construction be properly financed without gifts from two loans with corresponding withdrawals?

For gift tax, only the land and the debts taken over related to it initially matter. Everything that happens afterwards (house construction, increase of the mortgage) no longer matters, at least not for gift tax.

What can still be "risky" without marriage, as said, is inheritance tax for the case none of us wishes for.

If you mean to carry out a sham transaction to avoid gift tax - that would be tax evasion.

No, I claim that in a hypothetical case one can possibly structure it so that effectively no gift takes place and therefore with some skill no gift tax has to be paid. And that one might still have to pay considerable amounts of tax if it is not done correctly. Provided ownership structures etc. are chosen correctly.

No, I claim that in this fictional scenario one can possibly arrange it in such a way that effectively no gift is made and thus with some skill no gift tax has to be paid. And that one might have to pay considerable amounts of tax if it is not executed properly. Provided ownership structures etc. are chosen correctly.

Look, no offense, but it should actually be clear that no gift tax is incurred if the friend buys half from her at the real value, right?
For this insight, no consultation is absolutely necessary anymore.

For the real value or for the book value? And how does the man manage to prove to the tax office a certain portion of the loan, that is, a purchase price payment, when the loan is converted into a joint loan?