The petitioner would like the ruling for zakat on money saved for purchasing the marital house and for marriage expenses. The sum has reached nisab (the minimum amount upon which zakat is due) and one lunar year has passed over its possession.
If zakat is obligatory, how is it to be calculated for the previous years?

Prerequisites of zakat
• Zakat becomes obligatory when a person's wealth reaches nisab andhas been in his possession for one full lunar year.
• Zakat is obligatory upon an indebted person's wealth if the remaining sum, after deducting his debt, reaches nisab.
• The nisab must be in surplus of one's basic needs because money needed to fulfill a basic need is treated as though it were non-existent.

Definition of a basic need

In his meta-commentary (6/2), Ibn 'Abdin quoted the Hanafi scholar Ibn Malak's accurate scholarly definition of a basic need: "Anything that wards off certain harm such as food and drink, housing, weapons of war and clothing to protect from heat and cold.
[Basic needs also include] unforeseen expenses such as a debt which needs to be paid from the nisab to avert imprisonment (which is tantamount to harm), craft tools, household furniture, riding animals and books for the seekers of knowledge who equate ignorance with harm. The sum of money needed for any of the above needs is considered as though it were nonexistent. This is congruous to using water to quench one's thirst instead of using it for ablution; the water is considered nonexistent and, in this case, it is permissible to perform tayammum."[1]

Ibn Malak further elaborated that no zakat is due upon the sum of money reserved for a basic need which has been in a person's possession for one lunar year. Ibn Nujaim objected to this opinion in his book Al-Bahr Al-Ra`iq (1/222) and said, "The opinions expressed in Al-Mi'raj and Al-Bada`i' maintaining that zakat is due on money whether it is kept for savings or expenses, are antithetical to Ibn Malak's."
After quoting the previous scholarly opinions, Ibn 'Abdin said, "Since Ibn Malak's words are consistent with the apparent meaning of other [Hanafi] texts, it is fundamental to reconcile the opinions by interpreting what was mentioned in Al-Bada`i' and other books to mean that if a person possesses a sum of money which he needs to meet his needs and one lunar year passes over its possession, he is to pay zakat only on the remaining sum (if it reaches nisab) even if he intends to spend it in the future.This is because he does not need to spend the remaining sum on his basic needs at the time when zakat becomes due, in contrast to when he needs the remaining sum to cover his basic needs."

The ruling
Based on the above, no zakat is due on the money saved for purchasing the marital house and for covering marriage expenses because it falls under expenses for one's basic needs.

Allah the Almighty knows best.