Determining Proper Footing Depth
The footing is the support for the entire structure. Unless the property is located in a flood zone or in an area know to be problematic, in Florida we typically rely on either 1] SPREAD FOOTINGS and stemwalls, if required, meant to retain and reastrain the soil they contain or 2] a SLAB ON GRADE with monolithic footings at the slab's edges and under any point loads.
In Florida, we typically locate the BOTTOM of our footings a minimum of 12" below the virgin ground. And while it is true that we live in a relatively flat state, there are absolutely areas where the "lay-of-the-land" is hilly. Good building practices REQUIRE the surrounding land be evaluated before merely relying on the "12" below virgin grade minimum standard"! If you are intending to build on hilly land, the "12" minimum" is OUT THE WINDOW! In this situation your 12" footing depth is based on the LOWEST GRADE REASONABLY AFFECTING THE PROPERTY.
Soil migration, both surface and sub-surface, absolutely must be considered. There are several factors that cause soil migration. Some of the more common ones are 1] The natural effect of gravity is always present, 2] The surcharge of the pressure of the intended building itself and 3] WATER, both surface run-off [think rain] and sub-surface water [super-staurated soil caused by continuous rains, plumbing leaks, etc.].
Evaluate the land. Determine the elevations of 1] the road, 2] the undulations of YOUR property, 3] the surrounding properties and their affect on your property and 4] any bodies of water, natural or man made. After determining these elevations, properly consider their cumulative effect on YOUR desired structure location and set the height on your finished floor appropriately higher than the likely height of any water. Once you have the location and the LOWEST finished floor height determined, you can then evaluated what type of foundation system is appropriate for the circumstance AND - - HOW DEEP TO SET YOUR FOOTING! No "reasonable" amount of money spent in the future can correct a poor decision at this ultra-critical juncture of the construction process!!! Build with your finished floor high enough and your footing low enough. Spend the time, spend the money - build it properly while you have the chance!