Depths and sizes will be dependant on ground conditions...
In general dig out to 100mm. Use shuttering to take the level to 110mm, 10mm above ground level.
Then replace with 60mm of compacted type 1 as a sub base layer then put a DPM sheet over the top.
Add 50mm of poured concrete.
For large bases it is worth including reinforced mesh sheets to help avoid cracks and keep the structural integrity.
ALWAYS use DPM damp proof membrane and or DPC damp proof course rolls to separate the base layers from the building. This will ensure minimal moisture ingress up through the walls and floor.