Buried Electrical cable

I need to supply power to my back shed approx 40m away.
I know i need to to go down 500mm but part of the way will be behind a a retainer wall. My question is: if i continue the depth from the house (500mm) behind the retainer wall it will be 1.2m buried. Can i raise the depth of the cable to 500mm when its behind the retainer wall or do i need to continue at the depth i started from at my house level?

the top of the conduit needs to be minimum 500mm below grade
which is why most people dig 600mm deep

just needs to be 500mm below the ground surface above it, you could surface clip to retaining wall alternatively. masked sure your sparky is happy. Change levels with junction boxes and risers