Retaining walls of certain heights and where they are related to certain structures must be engineered according to council requirements. Each council is different and your council will advise you on the specific needs in your area.
Retaining walls need proper design in order to function correctly and keep the earth back or the structures on top of the earth sound. This means drainage and the selection of proper materials.
Wise Drafting can design and draw your retaining wall plans which will need to be taken to the engineer for completion and signing then put into the council for approval.