"Geoff Mills" wrote in message
...
Hi, I'm planning to cut into the ground floor power cable ring and
extend it into the adjacent, integral garage. ...

Is it common practice to return the cable down the same conduit that
it came up in, ...

It is acceptable, but if you are using twin and earth cable, rather than
individual conduit wires, you may find it a bit of a tight fit around elbows
in 20mm conduit. I prefer 25mm conduit if I am running two 2.5mm T&E cables.

Colin Bignell