There are two conditions that determine how high you can build a garden retaining wall. The first condition is that the height restrictions set by the local council or state government, and the second is the physical capacity of the retaining wall block, or sleeper wall, or other chosen material, that you wish to use.
Retaining wall producers will specify the maximum height that products can be constructed to without engineering. These are often known as ‘gravity wall heights’ because the wall gains its structural integrity, and plays its retaining function, by using its mass weight to keep the soil behind it. Sometimes, you might still need an engineered design to build below the council height limit — it all comes down to the capacity of the retaining wall system you use.
At the end of the day it’s not advisable, nor generally permitted to build above the manufacturers or engineers specified heights. In doing so you may be in breach of local or state planning laws and liable for any injury or damages caused to the public or environment.