If the JDA explicitly excluded the roof from the saleable area and this was maintained in the sale deeds to third-party flat buyers, the landowner's claim to the roof is valid.
Verify if the sale deeds of the 0-9th floor buyers explicitly stated the roof is exclusive to the penthouse.
Under the Real Estate (Regulation and Development) Act, 2016 (RERA), roofs, staircases, and terraces are explicitly defined as common areas.
Every apartment owner holds an "undivided interest" in these common areas. This means no single party, including the original landowner or builder, can legally claim exclusive ownership or block access unless very specific conditions are met.
Courts have repeatedly held that any clause in a sale deed or development agreement that contradicts the law—such as a builder or landowner retaining exclusive rights to a common terrace is null and void.
The municipal-sanctioned building plan must explicitly mark the terrace as an "exclusive use" area for a specific unit (e.g., the penthouse). The terrace must be accessible only through the private unit and have no common entry point (like a shared staircase). The RWA/Society must formally agree to this arrangement in its bylaws.
Upon formation of an RWA or Co-operative Society, the builder/landowner is legally required to hand over the entire building complex, including all common areas (terraces), to the association. Regardless of ownership, the roof must remain accessible for maintenance (water tanks, lift rooms) and emergency fire safety as per the National Building Code. Even if the landowner retains the right to construct additional floors (FSI), they usually cannot block current residents from using the existing roof of the topmost floor.
You can approach court of law with a suit for permanent injunction against the land owner/10th floor owner to restrain him from obstructing the access to the terrace for the essential purposes.