A Fully Integrated Structural Building System has a pair of pre-stressed multi-stemmed concrete floors. The pre-stressed multi-stemmed concrete floors are adapted to contact each other on one parallel side edge to form peripheral edges and a central joining edge. A part of a building is coupled to a pre-stressed multi-stemmed concrete floor. The building is constructed with a primary structural frame and a secondary structural frame separated by a rigid foam spacer. The spacer is a Class I non-combustible closed-cell foam. The building is strong, energy efficient, non-combustible and easily transported from the factory to the building site. The building is Green and constructed only with renewable and recyclable materials.