later inheritance in patchwork family

You are just constructing unnecessarily complex scenarios here. Then questions arise, when was it built before or after the wedding, what were the preliminary declarations, at what value was the property contributed, how much is everything worth then, etc. It is not a clean solution. And he doesn't come out better either, because then he is dead. The cleanest solution would simply be that he buys half of the property. And both are registered together in the land register. If other solutions are desired beyond that, a notary should regulate it testamentarily if possible.

Then questions arise about when construction took place, before or after the wedding, what the liabilities were, at what value the property was contributed, how much everything is worth, etc.

I assumed now that they were already married at the start of construction.
Tracking liabilities historically should not be a problem.
You also need to determine the value of the property for your idea if the OP is supposed to buy half of the property.

And he doesn't come out better at all because then he is dead

That's a case that admittedly was probably the main concern of the OP. But separation/divorce also plays a role and was also part of the initial question.

The cleanest solution would simply be that he buys half of the property. And they build jointly with both in the land register. If other solutions are desired beyond that, then a notary should handle it testamentarily if possible.

Probably, but it costs money. And you can't avoid the property valuation either, because the price for the property somehow has to be determined and the notary fees are based on that.

But yes, it is definitely cleaner than doing nothing at all or just getting married. True.

I also consider buying out to be the safest and best solution. It doesn't have to be 50%, he can also just buy 30% of the property. Then later he would only own 30%. If the children’s arrangement stays the same (she 2, he 1), that would also fit well.

Even if part of the property is bought off: particularly in the event of death, a regulation is also needed when marriage occurs. Otherwise, the children of the first deceased partner are disadvantaged. They only inherit half of the half that belonged to their parent - the other quarter goes to the surviving partner and ultimately to their children, so that in total 75% is inherited. If this is not desired, a will is necessary.

I believe you need to formulate very precisely what you want to achieve here. What do you consider the fair solution you are aiming for? Then get advice accordingly. A will is needed, as well as a regulation for the case of separation.

I think you need to be very precise here about what you want to achieve. What do you consider to be the fair solution you are aiming for? Then get advice accordingly. A will is needed, and an arrangement for the case of separation.

I can only agree here from my own experience! For the notary you trust, it is easy to put your wishes into a contract. And even if it is currently raining rose petals, things can very quickly turn out very differently.

Mr. Pastor is at a loss for words?