You can find the studs, by finding the nails/screw heads with one of those rare earth magnets. Mark where it sticks, then the line of the stud should be obvious. The ladders come in various types, some swing up and over in two sections and need clear space height to land - others can fold up just on the door.
I can make the 4 section ladder fit where existing hatch is so that's saved me a load of work!.