I've used the very loftiness of the hi-ceilinged spaces in constructing a number of LOFTS, or additions, within these lofts;   all capture some of the CUBIC feet and convert it into SQUARE feet....to do this sucessfully FOLLOW THE CHARACTER OF THE BASIC CONSTRUCTION:  IF IT APPEARS INDUSTRIAL THEN USE COMPONENTS THAT EMPHASIZE; LIKEWISE  picking up on other features is another way toward the art of the addition, for example, in many reclaimed older buildings the floor may be wood distressed by years of use, then varnished or painted without hiding its prior use.   This attractive feature may make cedar beams on the new LOFT appropriate.   Follow through with wood handrails over more simple plain surfaces as sheetrock.  Pick a part of the room you're working within as the least useful and least attractive...that, I believe, will lend itself to the highest return value for the effort involved because it will be the most changed by your project.  Keep the view open towards the windows and generally use the space under the stairway...a closet or small bathroom generally works well.or maybe design an office area...since the desk doesn't need the height.