Thanks Jim! No, not exposed, covered with shiplap siding. I wanted to keep this job a little quicker and easier, since the guy I am building it for needs it done soon. And I figured that this would be sufficient given that no floor is attached.

I do have one other question. As shown in my diagram, I have the back sill member having the top lap, while the shorter side sills will have the bottom laps. I did not know which configuration (as I have it, or the other way around) would be more stable.

It would seem that it depends on how the load is transferred, and any predicated failure modes. I figured in my design, the local failure in the mortise area might be worse than having that whole back "bent" overturn. I am not sure about that, since the whole back "bent" could rock the back sill. But I was thinking of using some strong tie plates between sills and posts as well.