Lego style building recently started to look pretty good. It also has potential of being extremely cheap. It may be even possible to do rearrangements (moving, shortening, enlarging walls etc) later if you feel like or reusing your old house "bricks" to make new one.
This is actually a very good idea. I would personally love an easy reconfiguration that I can pre-design myself: a young kid --> cut off a master bedroom chunk for a semi-detached room; older kid -> his own room, (teen -> full enclosure, enter from the roof :) )
If it is relatively quick/automatic one could reconfigure before/after visiting friends.
