Do I need approval for a retaining wall?

A retaining wall holds back soil that would otherwise move, and the load it carries rises sharply with height, particularly in reactive clay after rain. That is why every jurisdiction sets a threshold, and why the threshold is not the only condition.

Height is not the only test

• Surcharge. If anything sits above the wall adding load, a driveway, a parked car, a building, a pool, or even another wall stepped above it, the exemption typically falls away regardless of height.
• Proximity to a boundary, since a failure affects the neighbour and their land is often what is being retained.
• Whether it supports a structure or a road.
• Drainage behind the wall, which is the single most common cause of failure and is often what an engineer is really specifying.

Drainage is the part people skip

Water building up behind a wall generates hydrostatic pressure that most walls are not designed to resist. A properly built wall has free-draining backfill, a geotextile to stop it silting up, and an agricultural drain taking water away. A wall built without those may stand for years and then fail after one wet season.