What is the amount on which zakat is due from money saved for building a house, if part of it has completed a hawl (lunar year) and reached the nisab (minimum threshold), while the other part has not yet completed a hawl, knowing that the construction is not yet complete?

Zakat is obligatory on money that has reached the nisab and a full lunar year (hawl) has passed over it. So, it becomes obligatory on five thousand dollars if a full year has passed over it. It is easier to pay zakat on all the money at once when the hawl of the first money subject to zakat arrives, such as in Ramadan. The fact that the amount is for building a house does not affect this. As for what was spent before its hawl, there is no zakat on it.