24 CFR § 886.325 - Overcrowded and underoccupied units.
(a) Change in family composition, family's notification. The family shall notify the owner of a change in family composition and shall transfer to an appropriate size dwelling unit, based on family composition, upon appropriate notice by the owner of HUD that such a dwelling unit is available. Such a family shall have priority over a family on the owner's waiting list seeking the same size unit.
(b) Change in family composition, owner's responsibilities. Upon receipt by the owner of a notification by the family of a change in the family size, the owner agrees to offer the family a suitable unit as soon as one becomes vacant and ready for occupancy. If the owner does not have any suitable units or if no vacancy of a suitable unit occurs within a reasonable time, HUD may assist the family in finding a suitable dwelling unit and require the family to move to such unit as soon as possible.
(c) HUD actions if appropriate size unit is not made available. If the owner fails to offer the family a unit appropriate for the size of the family when such unit becomes vacant and ready for occupancy, HUD may abate housing assistance payments to the owner for the unit occupied by the family and assist the family in finding a suitable dwelling unit elsewhere.