All perfect praise be to Allah, prayer and peace be upon Prophet Muhammad and upon his family and companions.
First: Zakah isn`t due on the value of rental apartments, but the landlord is obliged to pay Zakah when his money reaches a Nisab (i.e. equals the value of 85 grams of gold) on which one lunar year has passed. As regards apartments set for sale, their value is estimated and Zakaah is calculated accordingly.
Second: when a person doesn`t pay the Zakah due on his wealth, that Zakah becomes a debt from which he/she is relieved only by paying it off. Therefore, if he/she dies, it becomes a debt that the heirs are obliged to pay off even if it covers all the inherited capital. Al-Imam Al-Nawawi (May Allah be pleased with him) said in the book,[Al-Majmoo]: “ Whoever was able to pay the Zakah due on his wealth, but didn`t, and died, Zakah must be paid from his estate because it is a right that he should have given while alive, and being dead doesn`t absolve him from paying off that debt. Evidence supporting this is the Prophet`s Hadith: “ Allah debts have more right to be paid “. {Narrated by Bukhari&Muslim}.” Accordingly, your father, may Allah have mercy on him and accept his repentance, should have given Zakah for all the years during which he failed to carry out this obligation, thus completing his repentance. Since he did not do so, the heirs should give Zakah on his behalf for all those years.
Third: if the heirs knew the amount of Zakah due on the deceased, or were able to estimate it, they are obliged to pay it. Nevertheless, if the heirs had doubts about the dueness of Zakah on the deceased, and couldn`t verify that, but it seemed that their father used to pay the Zakah-even by payments-they aren`t obliged to pay from the estate since doubt doesn`t establish a basis for a ruling.
Fourth: current charity is a Waqf (endowment), but giving money to individuals is a charity , but not a Waqf. Charity differs from Zakah as it is permissible to give the charity to a kin even if he/she was an heir. And Allah Knows Best.