If the access is located in the ceiling the minimum unobstructed headrom above the opening must be 30 inches measured vertically at some point over the access.
Attic access hatch minimum size.
The rough framed opening must be a minimum of 22 inches by 30 inches.
The access opening should be located in a hallway or other readily accessible location and minimum 22 x 30.
Minimum vertical height of 30 inches measured from the top of the ceiling framing members to the bottom of the roof framing members.
The 2007 california building code requires a minimum attic access opening of 20 inches by 30.
Minimum opening size the 2012 international residential code requires an attic access opening for attics with an area greater than 30 square feet and a vertical height in excess of 30 inches.
An attic access opening runs between the ceiling joists whenever possible.
The compact attic ladder requires a minimum height of 28 and minimum depth of 44 inside the attic above and behind the opening.
If the attic space is smaller than these specs then an access hatch is not required.