The frost line, also called frost depth or freezing depth, is most commonly the depth that the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. You have to have freezing weather for a while to have a deep frost line. - wikipedia.com
If you have a 28" frost line, you will need to build your base slab 28" below grade (“Grade” means “Ground Level”). If your frost line is at 28", you slab will need to be about 36" thick.
As for the need to building a base foundation… that is going to be your call. Several city / states require any load bearing structure be built on a frost line slab. Others city / states have no written law regarding frost line construction, so a standard 6" slab on 4" of gravel is fine.