Taking a page from the book of ‘lean,’ I would look for the least used floor space. An interesting exercise is to track your daily activities by time and function. What room do you use the most/ least? What are the functions? Take bedrooms, for example. In most cases, they have one function – sleeping - and other nocturnal activities. For this, you need a bed. You also need closet space, and dresser or two. The room can be designed around these items, with enough space for circulation. Gone is the master bedroom with the extra sitting room area, and all that extra open floorspace – good for waltzing around – a highly unlikely activity.
Certainly there is much fat to be carved off from building floorplans. Not just shrinking existing blueprints, but redesigning with the human as the central focus. Because we aren’t really designing walls or floors, we are designing space.