I have a tri-level house that has a sub-level attic access door that is 1 foot wide by 2 feet tall. Behind that door is an excellent area for a kids hidden playroom. I want to wall it in, and add flooring. The flooring rafters are actually 3 feet from the actual second floor (serves as the kitchen ceiling), but I have roughly 6 feet to put a door upwards to the apex of the roof (attic). This hidden door is in a linen closet about 1 foot deep. How do I put in a bigger door? Do I need to support the beams somehow when I take out the 1x2 access hole that is there? How do I cut the new frame?

 

Stick your head inside and look up the back of the wall behind the door. Look to see if there is a header over the opening.

If there is a header, you would need to cut it out and also any 2x4's that fall on top of the header higher (cut them at the height you want to place the new header). Then you'd install longer trimmers (the 2x4's that are underneath the header). You would then be ready to cut out the remainder of the drywall flush with the rough opening.

Since you say the opening is 1' wide, I would just assume that the door is located between 2 studs. It would be simplest to keep it that way. If you wanted to remove any jamb they installed between the studs to get yourself a little extra room, that would be okay. The distance between 2 studs is normally 14 1/2". Cutting the opening taller would not be a problem, provided you don't have any electrical lines running back there.

For an access door like that, a 1x2 face frame installed onto the wall and then a simple cabinet door on top works really well, and doesn't make the opening smaller. Since I assume this is attic area that could have mice, and gets hot & cold, you would probably want to seal the back of the cabinet door with some sort of permanent weatherstripping.