Is buying gold for savings in order to build a house considered hoarding, and is zakat obligatory on it if it has not reached the nisab and a full year has not passed?

There is no objection to hoarding money if its Zakat has been paid. The blameworthy "Kanz" (hoarded treasure) is any money on which Zakat has not been paid. Any money on which Zakat has been paid is not "Kanz," whether it is buried or not. Zakat is not obligatory on gold unless it reaches the Nisab (approximately 85 grams of pure gold) and a full lunar year has passed on it.