GST On House Sell

I was planning to buy an apartment.I got a good option but not sure if I should proceed.The builder is quite reputed and associated with mostly large projects.The realtor told me he is building the house jointly with house owner.As the house was currently under construction I asked for if it will attract any gst.He told me currently he is developing in collaboration with owner which is not registered with GST.so as there is no registered gstin is not there,then no need to give gst.And also he asked me to apply for a loan as gst registrations does not affect loan application.The apartment has only 8 flats.My questions are 1.should I buy from the realtor as he is not registered. with gstin number.Though he insisted all the paperwork is perfect.So loan wont be a problem.And the plan is already approved 2.for small apartment (8 flats) is gst is applicable? 3.after agreement can he ask for gst from me? 4.Now please let me know if an house owner building /adding another floor in his house and directly sells the same to me,will that attract Gst.What are the scenarios where under construction house sale does not attract gst?