Is it permissible for a father to assign to each son the floor he built in the building, out of fear that the husbands of his daughters will demand their share after his death? Is that considered unlawful (haram)? If so, how should the building be divided in a way that pleases Allah, in the event that the daughters demand their inheritance?

The way out for your father from falling into what is forbidden is for both of you to give him what he helped you with in building the floor and the price of the roof, or the price of the apartment's share of the land, and each of you owns his own floor. Or, he should give your sister the same as he gave you, so that he does not favor some of his children over others, because it is authentically narrated in Al-Bukhari and Muslim and others that the Prophet, peace and blessings be upon him, said to Bashir: "Did you give all of them the same as you gave him?" He said: "No." Then he said: "Then do not make me a witness to it, for I do not witness injustice."