I do not believe any such constraint exists that disallows a ground floor occupant to get the same UDS as those living in other floors if his flat area is the same as those of other flats on other floors. It is mainly dependent on the flat area.
What is one's undivided share in land is matter of negotiation. In case, no number is pre-decided, then it is assumed to be equal.
Generally in market, no FAR is used in basement and hence basement is sold along with one of the habitable floors (Ground/ First/ Second/ Third). In that case, normally undivided share of land is slightly higher then other floors.
In case FAR has been used in basement, then basement can be sold/ registered separately and then undivided share is matter of negotiation.