For verticle expansion, it would be worthwhile to look at the traditional german platform framing system. Each floor is a seperate frame, set on top of the one below. Posts are short while plates and tie beams are long. There is some info on this in the Guild's "Timber Frame Joinery and Design, Vol 2"- German Frame Typology, p222.

With proper planning and head scratching (and a big enough crane), you could design the entire roof to be removed as a unit (or units) and then re-placed atop one or more new levels.