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Boundary retaining wall

Need specs to extend existing boundary retaining wall. You can see in photos that I want to excavate and retain the small grass triangle. Back boundary is onto existing property. Other fence bounday is to common land with nearest road 4 m from boundary. Existing back boundary retaining wall is less than 1 m. It is also less than metre from boundary fence. Does this mean it has "surcharge" and need a building consent? Does that mean it also has to be 12kpa specification?