landscaping questions

Hi everyone
I have questions regarding landscaping and retaining walls, and would like some advice please thanks. we will be reaching Practical Completion inspection soon but wanted to start landscaping soon after handover. this is our first build so I am confused on the next step to get to Final O.C. The builder used private certifier for us throughout the build. I tried calling local council, but they advised us to contact the private certifier the builder used.

the builder lodged our DA through local council with simple landscaping plan. do we have to follow that landscaping plan lodged through DA prior to construction? are we allowed to make any slight amendments from the plan such as changing some part of the grass to concrete? if that need a new landscaping plan, do we lodge that through council ourselves or provide to private certifier and they then lodge this for us?

we also will be having retaining walls to both sides and rear of the house. the height will be about one metre height max. do we need engineering drawings plan from the engineer company for this before installing the retaining walls? do we have to lodged the retaining wall plan through council or provide to the private certifier and they then lodge through council?

could you share what is the cost the private certifier normally charged for all the process to get the final O.C? and do we have limit time frame to complete all the landscaping to get Final Occupational Certificate ? thanks

thanks a lot