I want to replace the metal soil pipe at the side of the house with a plastic one.

Fairly easy job, worst part is breaking small amount of external ground to expose fitting between cast iron and clay connection under floor. Once exposed and iron removed, clay cement used to bond between old clay and new plastic. From there the only way is up, replacing all with plastic, clips every 1.8m vertically, 1.5m horizontally. Alternatively, you can used a rubber iron-plastic adaptor at low level and work from there.....