Yes, the door was flat and pre-hung. It has about a 2" metal frame that sits about 3/8" proud of the sheetrock wall. The stiles on either side of the door are 3/4" by 4". After adding the 1/4" plywood to the wall up to the metal door frame, the metal frame was 1/8" proud of the plywood. I rabbeted the stiles 1/8" by 2" so the stiles would sit tight on the plywood. No need to modify the casing.Thanks for the info, I assume the metal door had a flat surface to work with correct? Was it a pre-hung door? If so did you have to modify the casing to accomodate the thickness of the wood panel?