Is zakat obligatory on a house that the father bought with the intention of preserving money and making a profit, then his intention changed for it to become a dwelling for his son, with the son being uncertain about his residence in it due to the possibility of selling it and buying another, and if it is obligatory, then upon whom is it obligatory and what is its amount?

If a father gifts a house to his son for residence, and the son does not intend to sell it, or he intends to sell it and buy another house, or if the father did not gift it to him but the father does not intend to trade with it, then Zakat is not obligatory on this house, neither for the son nor for the father. This is because the default is that Zakat is not obligatory until the intention is purely for trade.