I am going to use a somewhat extreme example to illustrate something here.
I have seen plenty of examples of very old timber frames where the walls have absolutely no direct support whatsoever from the foundation. These are cases where the first floor of the building is built of stonework -so it serves as the foundation- and the second floor is timber framed, but jettied on all four sides well past the stone wall.
Now here is my point. I believe it is possible to have an outboard enclosure system that has no direct support from the foundation at all. The frame would sit on the foundation and the the shell would be 'jettied' out beyond. This could save tremendously on the cost of building an oversized foundation, and also there are 2 huge advantages that this makes possible.
The exterior shell can go down along the foundation past the sills, this can in turn provide excellent insulation in a very hard to insulate spot. It also totally isolates the sill from water ingress. The only 'sill' involved in such an outboard system would be a piece of wood enclosing the bottom. This would obviously have to be used with a wall system especially adapted to this.
