I had imagined that the resize tool (keeping the aspect ratio the same) would permit the creation of a smaller room on which to auto build the room foundation wall (supporting the double skinned cavity wall).

Apologies,

that should actually read as:

I had imagined that the resize tool (unchecking the "keep aspect ratio of" radio button and reducing Width/Height by the same amounts) would permit the creation of a smaller room on which to auto build the room foundation wall (supporting the double skinned cavity wall).