If you notice, my second floor on the north side and part of the east side facade is offset from the first floor facade walls.I have in mind(because of my design and the 1800sq.ft. limit) to make
it from:Brick(35cm) and if it is possible a small fill rammed earth(15cm) just for facade exterior look.In the brick wall there will be a wood columbs that will handle the roof. And a second option is to be build from wooden wall.
Anyway if this materials doesn't fit in your project, all walls may be build in one line from first to second floor like i show in my last 3Drender with a few correction of the plans:first floor-smaller living room(front of the tv space); in the second floor-bigger bathrooms and bigger exercise room and master bedroom.
Best wishesh and thanks for the great competition!