Bedroom size is calculated by looking at the age and sex of the family members.
Children of the opposite sex will share a room up to age 10. Children of the same sex will share a room up to age 16. Therefore a family of four with one adult parent (first room), two children under age 10 (second room), and one child 16 or over (third bedroom) would qualify for a three bedroom unit. If the family of four included an adult parent (first room), a male child of 11 (second room), a female child of 13 (third room), and a child of 17 (fourth room), they would qualify for a four bedroom unit.