Stilt area in a building

I have a query relating to the stilt area in a building. Please help me out with this. As regards a stilt in a building, does the builder have the right to sell only the stilt to some other party? Isn't the stilt, by law, a common propery of all the residents of a building and don't all the owners of flats in the building have a common right over it?

Thanks in advance.