ToolGuy is correct; you only need a box step 7" tall, you step on that and then on the upper floor. The box could be 4' wide if you want to cover the door, comfortably. Build the rectangle 6 1/4" tall, screw the back board to the wall and add the tread on top of the box. You could even round the ends of the tread to make it look a little neater. The tread should be made of a 1 X 12 board and make the box an inch less than the width ( front to back )of the tread.