short term capital gain tax on redeveloped flat

Ours is coop hsg. soc.situated in south mumbai.we have redeveloped the society on our own by giving private contract instead of appointing any builder under rule 33/7.Extra FSI was taken by existing soc members only by contributing the funds as per rate decided for per sq ft basis.the work started in 2008 & completed in 2015.possession given on March 2015. The original area was 290 sq ft carpet and the present allotted flat is 670 sq ft carpet area. We have contributed for the extra amount in phases and instalments as was decided by the society. We have recd MHADA Final O.C. by June 2016 & BMC O. C.is in the pipeline and may get in 3-4 months.if I want to sell this property & want to invest in other property do it attract any capital gain tax. we have been given to understand that one deal of Flat in the said building has already happened by paying necessary stamp duty etc and the agreement is registered with the stamp office & on the basis of the documents submitted by the buyer who has also taken a loan from a cooperative bank. pls note that we paid first advance in 2008 ,2009,2010,2013,2014,2015 on installments basis. kindly guide about this.