The rough framed opening must be a minimum of 22 inches by 30 inches.
Attic access hatch minimum size.
Minimum vertical height of 30 inches measured from the top of the ceiling framing members to the bottom of the roof framing members.
These specs are per r807 1 of the international residential code irc and the residential edition of the florida building code fbc.
According to the 2006 international residential code r807 1 attic access homes with combustible materials in the ceiling and roof construction must have an access to attic areas that exceeds 30 square feet and with a vertical height of 30 inches or greater.
If the attic space is smaller than these specs then an access hatch is not required.
An attic access opening runs between the ceiling joists whenever possible.
For smaller attic openings with a minimum measurement of 18 x 24 you will need to measure the swing clearance inside the attic space.
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.
The 2007 california building code requires a minimum attic access opening of 20 inches by 30.