That's going to be so much easier and makes a lot of sense, wasn't too fond of your first plan, just have to take up more of that slab. The RW clay pip is running parallel to the building, so makes sense. If you trace it back can you locate the beginning point, i.e a RW down pipe. Just seeing where its easier to replace that section in plastic or having to 'T' into existing run. Would you need a rodding point or would the be happy with an access boss at the bottom of the stack? If former then you will essentially have to 'T' in twice.