Building on a firm foundation requires the existing grade to be raised due to several possible factors, building in a flood prone area, septic system elevation requirements, last of all which is standard at minimum is the elevation of the lowest slab being 18" above the crown of road. 

Raising the elevation of the lowest slab ensures positive drainage away from the home which is always a plus.

A firm foundation always comes down to prepping the soil conditions and absolutely verifying that the building area is completely clean of organic material.

Removing all tree and shrub roots and existing grade to the predetermined depth ensures that the final root rake is free of all organic and deleterious material.