When a Family Live in their Own house they won’t receive any Rental Income from that “House Property” , so Income from Self occupied Property is taken as Zero for Calculating total income, and no standard deduction of 30% is allowed as deduction.
Reference Section: 23(2) of Income tax Act
The Section says that when a house is occupied by owner or his family then the annual value of the House or part of Property shall be taken to be Nil.
Provisions of the section do not apply if the Owner earns any Income from the Property either directly or indirectly.