I am planning a bungalow loft conversion, and want a bathroom on the new first floor.  At the same time, I am splitting an existing ground floor bedroom into two to make a new bathroom and cloakroom/wetroom.  Existing bathroom to be removed (to make way for the stairs)   I have had some building control drawings done which show a new soil vent pipe up through the roof.  It was suggested in a previous post that I could perhaps have an air admittance valve inside the building instead