Climb into the loft and examine the existing construction to see how the opening is made, in which direction the joists run, and whether the ceiling finish is of plasterboard or lath and plaster.
You will find that the opening formed by the builder is framed by two trimming joists between the main joists. The easiest way to increase the area is by removing part of one main joist and using longer trimmers. This almost doubles the length and width of your original opening, and will be sufficient for most purposes. Commence by marking out the area of the new hatch, by scribing a line across the joist and ceiling where it will be cut.
Temporary support must be provided for the joist that will be cut. This consists of one sturdy upright, with a bracing batten or plank run to the floor, at each end just outside the line of the new opening. Remove the door and lining of the original opening, and prepare the two new trimming joists. These are marked off by direct marking (you lay each length of timber over its intended position while you mark it) and cut on the outside of the marking lines to ensure a tight fit. Locate the heads of the nails that hold the existing trimmers in place, chisel some of the surrounding wood away so that the heads protrude slightly, then pull the nails out with pincers.
Carefully remove the moulding, if any, surrounding the underside of the opening, then prise the trimmers out. Now cut through the portion of joist that is to be removed. At one end you can use the existing opening to provide space for the downward thrust of the saw, but at the other end you will have to drill a series of holes, then chisel them into a slot, to provide space for the. saw blade. The portion of joist is prised away. Keep it-it can be used as the new trimmer which runs parallel to the existing joists. Carefully remove the rest of the ceiling material by cutting along the marked lines with a panel saw.
Do this very carefully so that you don’t create any broken edges. With luck you can avoid any re-plastering. Fit the new trimmers. The two short ones are skew-nailed to the main supporting joists, and the long one is skew-nailed to the two short ones. Run strips of battening, narrower than the height of the joists, round the bottom inside of the new opening, to provide a lining and a stop for the new door. This battening must be sunk to a depth that will bring it flush with the ceiling below. Any gaps between the battening and the ceiling can be filled with plaster; or you can run a moulding round the edges to finish it off.