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.
Attic access code requirements.
When located in a wall the opening shall be a minimum of 22 inches wide by 30 inches high 559 mm wide by 762 mm high.
Here s the full text of the code.
To convert your attic to living space you cannot provide access via a ladder.
What your attic probably does not have is a set of code compliant stairs.
Where the access is located in a ceiling minimum unobstructed headroom in the attic space shall be 30 inches 762 mm at some point above the access measured vertically from the bottom of ceiling framing members.
See the california mechanical code for access requirements where mechanical equipment is located in attics.
Buildings with combustible ceiling or roof construction shall have an attic access opening to attic areas that exceed 30 square feet 2 8 m2 and have a vertical height of 30 inches 762 mm or greater.
The rough framed opening.
Your attic may presently have a fixed ladder a pull down ladder or no ladder at all.