I'm not saying there's any rule for a situation like this (in fact I'm pretty sure there isn't, it's done on a case-by-case basis) but there was a similar situation here - the PCP was as old as the estate, and got built at the end of (but fully inside) someone's garden. They just put the DSLAM beside it, about three feet away, but slightly "forward" of the PCP and so fully in the pavement.
Edited by deleted (Thu 24-Aug-17 23:37:54)