Is Allotment Letter From Builder For Open Space Legal Document In Future

I'm planning to purchase 1st floor flat in Pune, the ground floor has Car parking and 1st floor is Podium floor where children play area and other amenities are available. property is RERA registered. There is open space in front of Balcony(approx 250 sq ft) for which builder is charging 25% of the space and saying he can't add the space in Sale agreement, however, he can give allotment letter for this 250 sq ft area. My questions are: 1) Will this allotment letter be a legal document in the future. 2) Can society object the use of this space . 3)When the building will go for reconstruction(may be 30-40 yrs later), can I claim for the said open balcony 250 sq ft land which won't be there in sales deed. Thanks in advance for your help here.