Buying a Pagdi Flat which is going for Redevelopment

Hi, a reputed and known builder to us, has given a option to buy a tenancy flat which is appx 500 sq ft. On redevelopment this will be appx 700 + sq. ft and this will take around 4 years. Mhada NOC is received and after IOD , building redevelopment work will start. This is expected to happen within 6 months. We have been informed to consult about Short Term Capital Gain applicable on such a proposal. There are several different views by professionals but the basic being that the cost of acquisition will be considered as zero, NIL and hence we might be subject to Short Term Captial Gain when we get the finished flat of 700 sq.ft. Hence there is a possibility that we have to pay STCG on the market value of the said flat of 700 sq.ft. This is very confusing for many we asked, as the STCG should only be applicable if we wish to sell the flat before 2 years after completion? The consideration towards buying of the Pagdi flat will be paid in cheque and will be registered with stamp duty being paid. Again Stamp Duty is applicable when we register the flat of 700 sq ft with the builder . Builder and Lanlord , both being well known to us are ready to co-operate with any letters that may be required for IT but we want to make sure if this is a clean and clear process. Can you please advise whether STCG will be applicable even though we do not wish to sell the flat and keep it for end use? There are several Pagdi flats on sale , which are on the cusp of redevelopment but we were not aware that there can be a Tax implication. Please advise, thank you