Figure illustrates a generic floor plan for a secondary bathroom…where there is not enough width between the two walls for the bathroom door and the casing to fit.
For this particular house, all of the doors to the secondary bedrooms and bathrooms were 2’-8” wide (32”). The exterior wall at the right side was 2×6 framing (5-1/2”). The door casing trim is 3-1/2” wide…with a ¼” reveal the door jamb and 1 1/8” air space around the door.
The architectural dimensions are face-of-stud to face-of-stud.
I use ¾” instead of 5/8” at the inside wall corners for the thickness of the drywall…because the drywall taping mud adds an 1/8th of an inch thickness (to get 5/8” thickness at the inside wall corners, all of the ripped pieces of drywall must have the uncut recessed edges…which seldom happens).
Figure shows the minimum width for this door and casing to fit…without having to rip to a narrower width one or both vertical side of the casing.
Figure shows that the dimension on the plans of 2’-2” from face-of-stud on the exterior wall to the centerline of the door…and 1’-8” from the centerline of the door to face-of-stud at the left side of the door (looking at the bathroom door from the bedroom)…is not wide enough for 39-3/4 inches to fit.
In this particular case, both sides of the casing on the right-hand and on the left-hand had to be ripped to a narrower width to fit…spoiling the appearance of the side casing detail compared to the full-size header casing above.
In hindsight, the dimensions on the plans should have been 2’-4” and 1’-10”. This would have provided a drywall reveal around both sides of the door casing.