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.