24 CFR § 3280.104 - Ceiling heights.
(a) Every habitable room and bathroom shall have a minimum ceiling height of not less than 7 feet, 0 inches for a minimum of 50 percent of the room's floor area. The remaining area may have a ceiling with a minimum height of 5 feet, 0 inches. Minimum height under dropped ducts, beams, etc. shall be 6 feet, 4 inches.
(b) Hallways and foyers shall have a minimum ceiling height of 6 feet, 6 inches.