I'm not sure what you mean by a raised center aisle. Do you mean 12' wide and 16' long shed with a 3' x 16' center aisle that is raised above the outer portion of the building's floor? Also how much elevation are you envisioning for the raised portion -- 2", 6", or what? There are so many variables in storage sheds. How high do you want it? Do you want a loft for extra storage? Small storage sheds with a loft tend to get goofy looking because the width and length appear disproportionate to the height unless it is located adjacent to another taller structure.