I should mention that the back of the house is a single story no more than 14 feet wide. Thanks, Patrick The sill serves as the transition between the foundation and the house frame. It can be anything from a simple 2x4 to a massive 12 x 12 timber, depending on how your house is framed. Bearing on top of the sill is the framing for your floor system and the exterior wall. Sills are particularly susceptible to decay because they are prone to getting wet from a variety of sources.
Rainwater splashing back off the grade can soak a sill that is too close to the ground.
Eight inches of ground clearance is the recommended minimum for new construction, but many older houses have sills right down at ground level. Also, any water that penetrates around door openings invariably gets to the sill, soaking it and encouraging decay. You will indeed have to find a way to support everything that rests on top of the sill in order to remove the decayed sections and fit new pressure treated material in it's place. How you support the load depends on the specifics of the house framing. I'll assume that it's not a massive timber frame, as these are usually found on houses earlier than the mid 1800's.