Builder asking to sign document at time of possesion

At the time of agreement, you see the proper permission is there for building plan from the municipal authorities and fire authorities etc. or not ? You must get Occupation certificate from the municipal auhorities but not from the builder. so that you will know the authencity of the builder agreement

1.  NOC is required from all the flat owners.

2.  No.

3.  Occupation certificate will be iuused by the Municipal authorities.

4.  Check whether there is a permission for the construction of further floors. 

Even if the builder is ready to give possession letter, without occupancy certificate from the competent authorities just mere possession is not valid. The authorities have to inspect the building for completion of construction and after confirming the safety to occupy by the incumbents, they may issue the certificate.It is the duty of builder to procure occupancy certificate.

The new agreement by the builder forcing the flat owners to sign may be for the construction of further floors unauthorized so that he can shield himself in the event of any untoward incidence during the process of new construction. 

There is no force on you to sign any such agreement, even then you may take a legal opinion about this agreement from a local advocate and confirm the legality of such an agreement.