Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
You do not have to pay the Zakah for this flat if you want to live in it or to rent it in the future.
Al-Nasa’e reported that Abu Hurairah narrated that the Prophet (Sallallahu Alaihi wa Sallam) said: “A person does not have to pay Sadaqa (Zakah) on his horse or his slave”.
The flat you want to live in is similar to the above and if you sell it, then you pay the Zakah after a whole lunar year passes and you still possess the Nisaab from that amount (equivalent of 85 grams of gold).
If you want to sell it, then the Zakah should be paid on its value after having paid the debts, but if you rent it, then you have to pay the Zakah on its benefits (income from rent) after having paid the debts, and after a whole year passes and if the amount reaches Nisaab (the minimum amount liable for Zakah). The Zakah is as you have mentioned 2.5%.
Allah knows best.