If a ownership building has common wall with a Cess building can we redevelop together using 2.5 FSI

Hello, I live in an Ownership building, but I have a cess building touching one of the walls of the building. This was done so that the ownership builoding could be built in the same plot. Now the landlord of the cess building want to redevelop both the building using the 2.5 FSI. What are the procedures for the same. Can we use his FSI to redevlop the ownership property

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