Slab for garden building

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Circa 3.25 by 3.25 by 2.75 high brick/block storage + disabled scooter end of drive, hence on garden. That would mean removing circa 500mm of soil as area above. At the bottom will be sand/clay/stones (digging for previous extension showed that). Clearance of 450mm is required all round plus a double garden wall (circa 500mm high).

So instead of digging for foundations for garage and garden wall could I lay a concrete base (thickness?) which would suffice as floor as well)? I'm concerned slab would not be strong enough for storage walls?

Thanks