Thanks everyone
To clarify a few points
The site has full planning and building regs approval , I think they only found the neighbour's septic tank soakaway when digging some test holes for a ground survey.
The new dwelling would be one meter from the rear boundary and the existing soakaway, the dwelling carnt be moved forward because there is a shared driveway that splits the plot in two and this carnt be moved either.
The new septic tank / soakaway for the new dwelling is on other side of the shared driveway so is no problem.
It appears the rule is in place because of the ground conditions, so I think the solution would be a pile foundation to account for this ? , assuming build control would allow it.