We have a foul drain that is routed past the existing conservatory, about a foot out, I would say. The conservatory is due to be replaced by a rather larger one, so we (and the architect) were looking to route it off at an angle and then back again so that it would not be under the new conservatory at any point. The builder we expect to use wonders why, his view is it would be perfectly OK to use the existing line and just bridge the new foundations over it.
If we did that I'd expect to use a single 5 or 6 metre length of plastic to replace the existing ceramic. That way any joints would be reachable without digging the floor up in the new conservatory.
Anything I should look out for if we go that route? The architect thought it should be OK, but, like us, likes to avoid doing that is possible.