State Environmental Planning Policy (Housing) 2021

Excavation for the purposes of a secondary dwelling or an ancillary structure must not be deeper than a maximum depth, measured from ground level (existing), of—

if located no more than 1m from any boundary—1m, or

if located more than 1m but not more than 1.5m from any boundary—2m, or

if located more than 1.5m from any boundary—3m, or

if the land is identified as Class 3 or 4 on an Acid Sulfate Soils Map or is within 40m of a waterbody (natural)—1m.

Fill must not have more than a maximum height, measured from ground level (existing), of—

if the fill is for the purposes of a secondary dwelling—1m, or

if the fill is for the purposes of an ancillary structure—600mm.

The height of fill contained wholly within the footprint of a secondary dwelling or an ancillary structure is not limited.

Fill that is higher than 150mm above ground level (existing) and is not contained wholly within the footprint of a secondary dwelling or an ancillary structure is limited to 50% of the landscaped area of the lot.

The ground level (finished) of the fill must not be used to measure the height of any secondary dwelling or an ancillary structure under this code.

Support for earthworks more than 600mm above or below ground level (existing) must take the form of a retaining wall or other structural support that—

a professional engineer has certified is structurally sound, including the ability to withstand the forces of lateral soil load, and

has been designed so as not to redirect the flow of any surface water or ground water, or cause sediment to be transported, onto an adjoining property, and

has adequate drainage lines connected to the stormwater drainage system for the site, and

does not result in a retaining wall or structural support with a total height measured vertically from the base of the retaining wall or structural support to its uppermost portion that is more than the height of the associated excavation or fill, and

is separated from any other retaining wall or structural support on the site by at least 2m, measured horizontally, and

has been installed in accordance with any manufacturer’s specifications, and

if it is an embankment or batter—must have its toe or top more than 1m from any side or rear boundary.