Which seller, without having sold the property, provides a complete stranger with their property as collateral. The builder throws themselves in front of the train, the property owner is stuck with the debt (they also have a house, but they don't want it)!?
You are talking here about two commercial owners, I only see one owner, namely the property owner (who cannot/will not sell their property yet). Besides, this is not about a partition (for example, a priority notice of conveyance can also be registered on a property area that is yet to be divided, and it can sometimes take several years – this is quite normal). Building on the property of parents or children is a completely different story, as a declaration of purpose for the land charge will be unproblematic there.
In the constellation mentioned by the OP, I guarantee you that no bank will finance a house on someone else's property, since the seller (who is not even a seller yet) will never see a reason to be liable for the house builder.