Inheritance for unmarried couple and subsequent construction project

The terminally ill live longer.

Is person C still legally competent? Then you could already directly gift a part to the third person or adjust the will.

True, but that's just as unromantic as thinking about the separation of the builders before the construction begins. But people do it anyway. Otherwise, in my opinion, C is no longer legally competent, although this has not yet been officially determined. Internal family arrangements regarding C's financial affairs are already in place. A will exists and clearly favors A, but it is unlikely to be changed.

Apart from that, A and B like to know their options before it actually matters.

True, but that is just as unromantic as thinking about separating the builders before the construction.

But that is a different matter than what you asked about.
Your initial post sounded like you wanted to pay as little tax as possible.
But whether married or not, an inheritance always counts as initial assets before the marriage, and is therefore not part of the community of accrued gains.

Just do it properly through official channels. Inherit and pay inheritance tax... done. After that, you can separately consider whether to sell half to the spouse or divide percentages in the land register.

If you want it romantic, just enter 50:50 during the construction and that's that.

That was just an example. Nothing is to be tricked, I only asked about the most sensible solution or about pitfalls. The plan currently is, as written somewhere in the middle: A inherits and pays the taxes due, B gets 50% on the land register after the marriage, and then construction takes place. How much more or less each pays in the end is settled between A and B.

B comes into the land register with 50% after the marriage

Then neither gift tax nor real estate transfer tax applies.