new bathroom above old

Hi - I am installing a new bathroom directly above an existing one. The new WC will be directly over the existing one, but 2.5 metres above it. It would be great if I could just extend the soil pipe up from the existing WC but I don't think that is allowed as the vertical drop is more than 1.5 metres. Is there some way I can use the existing soil pipe by extending it up from the existing WC or must I provide a new soil pipe going out to the drain?

New one on me. Blocks of flats have several loos all going into the same stack pipe, all one above the other.

if the stack is taller than 1.5 metres it just has to be vented thats what building control told me . perhaps thats where you got the 1.5 metres from