According to our local building inspector, they will not sign off a new septic tank installation - at all. They will allow repair etc. of existing ones, but new ones must be powered aerobic treatment centres.
Most likely due to either:
1) high water table meaning a soakaway isn't usable - so new units have to discharge to watercourse & package treatment plant. This is the EA's preferred option and national policy , so planning might reflect this. This way there is a distinct defined discharge point for them to sample if need be. Rather than an unsamplable diffuse source.
2) Nitrate vulnerable catchment - ie chalk or drinking aquifers. Package plants required to reduce nitrates in discharges.