Is zakat obligatory on money bequeathed for the construction of a residential property for the family and another for investment after the testator's death and before construction begins?

If the bequest is for heirs, it is void, based on the hadith: "There is no bequest for an heir," unless the heirs wish otherwise. The obligation of zakat on these funds is contingent upon each heir's share reaching the nisab (equivalent to 85 grams of gold) or being combined with other wealth. Zakat is then paid at a rate of 2.5% each year until the funds are used for constructing the property.