I have a similar situation to JMC - I only have a downstairs toilet
and I want to add an upstairs one.

Unlike JMC, I think my pipe slope should be OK, but I don't appear to
have a "soil stack" visible anywhere outside my house. My mate tells
me I must instead have an "air admittance valve" hidden somewhere near
my current downstairs toilet (either inside the house or outside under
some wooden decking I have). I've never seen it but it definitely
isn't upstairs (I have no loft).

If that is true, will there be a problem with the new upstairs toilet?
Will I have to move this "air admittance valve" to be higher than the
new highest toilet, i.e. the upstairs one?

Does the answer to this depend on whether I have a proper bog or a
pumper?

THank You.