الجواب وباللہ التوفیق

In the situation inquired about the Zakat will not be applicable on the apartment until it is sold. Once sold first the loan amount will be deducted from the sale amount. After that if the remaining amount is equal to the Nisab of Zakat then after a year the Zakat will be Wajib on it.

This must be clear that if you have already been صاحب نصاب (the possessor of amount equal to the Zakat Nisab) then it is not necessary for a separate year to pass on the apartment money rather it would be enough that a year would have passed on the other money you possess. In this scenario, it would be necessary to take out Zakat on the apartment money along with the Zakat on the rest of the money.

واللہ اعلم بالصواب