بسم الله الرحمن الرحيم
It is haram and unlawful to take interest based loan. The Holy Quran and Hadith sternly warn against it. The Holy Prophet Muhammad (may peace and blessings of Allah be upon him), has cursed those who take interest, pay it, write it and bear witness to it. (Mishkat: 244). Thus if you can manage, then you should avoid taking interest based loan. However, if you have to buy the flat then buy it directly from the builder in installments, though you have to pay extra amount compared to paying cash amount; but in case of buying in installments you should have to fix the price in the same meeting.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.