land share or old building size for joint venture

INRODUCTION An apartment was constructed at Chennai with five flats. This apartment is not currently usable due to the fact that floor area has submerged below road level. The details of the apartment are given below. Total land area : 4360 sqft. No of flats : 5 Land ownership of each flat owner : 1/5 th undivided share Flat sizes constructed : 700,850,725,750 and 1650 sqft. PROBLEM DESCRIPTION SINCE The flat is unused for past 2 years, it was proposed for joint venture construction through builder. Now builder is offering one flat each as per land share held by each owner. The problem is the owner who is holding 1650sqft flat is demanding two flats from builder even though he is holding one land share as others are holding. In this case what is legally correct wrt land share and joint venture agreement. Can that owner be directed to agree legally based on land share possessed by him. In this connection it is brought out that during initial agreement made in 1993 all owners were sold equal land as 1/5 th undivided share and then construction cost was collected based on building size.