On 13 May 2004 01:41:31 -0700, a particular chimpanzee named
(Bigus Dicus) randomly hit the keyboard and
produced:

I only have a downstairs toilet
and I want to add an upstairs one.

If you only have appliances on the ground floor, and you are in a
terrace or close to other houses on a sewer, then you may not have a
vent at all. It is acceptable to run a WC (and other fittings) less
than 1.5m above the invert into a vented sewer provided the branch is
less than 6m long.

If this is the case, then, yes, you would require a new soil stack to
the first floor. This could be vented with an air-admittance valve
internally. Provided you keep your downstairs WC, you can add a
pumped one to the first floor (subject to Peter Parry's approval).
--
Hugo Nebula
"The fact that no-one on the internet wants a piece of this
shows you just how far you've strayed from the pack".