Deposite of capital gains into CGAS Account

Hello, I have certain capital gains (eligible for us 54f), out of which i invested around 80% in the purchase of residential flat and paid instalments accordingly. the rest 20% i need to pay at the time of posession of flat i.e. after 1 year. for that do i need to deposite that rest 20% of capital gains into capital gains saving account scheme to claim exemption ? if yes do i need to deposite before 31st of march or before due date return filling.