What does your building code say? These "ccok book" requirements sometimes can be by-passed if there is a logical explanation as to why. For instance, suppose bedrock is found at one foot depth. Any half way smart building inspector will say you can set footing at top of rock, if you want to. Also some areas have deep deposits of clean sand, not frost susceptible. Even though the low moisture content sand commonly has very deep frost penetration, no heaving takes place. In that instance a code requiring deep footing grades can be modified provided a competent person explains why. So go back to your geotech and ask for a recommendation.
