Garage footings or slab?

[muchenje]

Hello
I wish to build a single weatherboard garage along a ditch line where one edge is already dug out as it forms part of the ditch about 500mm lower then ground level. Do I need to dig down to that level and infill up to floor height, or can I make a 'wall' along that edge using concrete posts & blocks up to floor height and throw a slab?