We have an interior wall that is mostly 2×4, but has 2′ of 2×6 to accommodate a DMV stack. We are  extending the 2×6 stud wall three more feet in order to get rid of the visual distraction, and add a 6″ deep medicine cabinet. This is a ranch home. The wall is perpendicular to the joists, so I would think it was load bearing. The bottom plate sits on floor joists, offset about 1-2 feet from a concrete block wall supporting it.  The new medicine cabinet is 24″ wide (22″ RO) and curious about how to frame around it. There is also a heat duct directly under that spot.  Since the RO is 22″ wide, could I just do 6″ studs at 24 OC?  Or do I have to frame it like a door (see below).  I assume we’ll have to cut out part of the top/bottom plate to replace them with 6″ lumber.  Framing is not my area of expertise and would appreciate any advice.