A perfect shape is a ball, next to perfect is half a ball, a dome, giving the ultimate ratio between floorspace and outside surface.
The dome either to be constructed as a geodesic dome built as a metal or wooden frame and sprayed with hempcrete, or hempcrete (reinforced by a metal grid) sprayed over a balloon.
A 13 meter diameter dome will have a height of 6.5 meters, a ground surface of 132m2 with a smaller usable surface at a second level of some 25 m2.
Depending upon the insulation values of hempcrete more insulation layers could be added to the outside.
I firmly believe a passive house needs to be insulated from the inside out, to create the ultimate climate in the living area, supported by underfloor heating, solar panels, heat pump and heat recovering system, .