another thought on the horizontal expansion is to keep the timberframes separate, and connect with a separately framed hallway, could be done stick style, then you don't have to worry about peeling off walls etc. Just build another frame beside the intial one an connect it.... the foundation may be the concern in this scenario, depending on your soil, it's not always that nice to dig close to an existing building.
I like the steel idea for horizontal growth, just weld er up eh???... might need some spark proof insulation
