how are you so sure that the builder is using the FSI of your plot on the adjoining plot for making construction?
you need get the proper information regarding utilisation of FSI on neighouring plot from the BMC
Also the affidavit is of no use to the builder because once the building is completed and OC is issued, then builder has to convey the land and structure to the society of unit purchasers
the conveyance will be as per the terms contained in the agreements for sale executed with the buyers
a simple affidavit taken subsequently cannot bind the society members. However the builder will use that as a defense in the pending deemed conveyance proceedings
also it is no excuse that the affidavit was given to the builder 'in ignorance'. That defense or excuse does not look plausible
it cannot be that all the society members 'ignorantly' made such an affidavit!
as regards the ongoing construction in the adjacent plot, first get proper info from BMC and if they do not reply then approach high court by filing a writ petition
check your agreement for sale as to what is stated regarding use of FSI of the land underneath the society building
if builder informed in the AGM that he will use the society's FSI on the adjoining plot, then what prompted the members to give that affidavit?!!!!