The message .com
from "deano" contains these words:

I need to add a toilet upstairs via an external stack. I'm also
thinking of connecting the downstairs toilet to this new stack, out
through the back wall and blocking off the current underfloor
connection. This is mainly because adding new pipework (for both
toilets) would be easier than trying to tap into the existing
underground one that goes to the downstairs toilet (I think?).

Where is the current stack and why is it not practical to plumb the
upstairs WC direct into that?

--
Roger Chapman