Site preparation for a BAL-rated build: defendable space

Defendable space is the zone around a building that is managed to reduce the fuel load available for a bushfire. The Planning and Design Code and CFS guidance specify defendable space dimensions and vegetation requirements.

The immediate zone (typically the first 10 to 20 metres from the building) should have minimal fuel: no shrubs, mown grass, and any trees pruned so canopies do not touch the building. Non-combustible mulch or hard landscaping is preferred over bark mulch.

The outer zone (typically 20 to 100 metres from the building depending on BAL level) can have more vegetation but should be managed to reduce fire intensity: canopy separation to prevent crown fire, no ladder fuels (shrubs that carry fire from ground to canopy), and cleared understorey.

Vegetation management in the defendable space is an ongoing responsibility. Annual pruning, grass mowing and leaf litter removal are all required to maintain the defendable space effectiveness.

In bushfire-prone areas of the Adelaide Hills and Fleurieu, a dedicated water supply for firefighting is often required at planning consent stage.

The typical requirement is a static water tank of 22,000 litres or more, with a specific outlet fitted to accept CFS appliance couplings. Some council rules also require a pumping capability so the water can be delivered to any point on the property.

The tank must be located where a fire appliance can access it easily, typically within 6 metres of the vehicle access. It must be marked with clear signage visible from the driveway.

For properties on reticulated water, the tank is usually still required as a bushfire backup because reticulated supply can fail during major fires. The tank water can also be used for property irrigation between fire seasons.

Confirm the specific water supply requirements with your council; they vary between council areas and by BAL level.

Fire appliances need clear access to the property to be able to defend the building. Access requirements typically include: minimum driveway width of 4 metres for straight sections, 6 metres at any corners or turns; maximum gradient of 12 percent (some councils allow steeper for short sections); a turning area at the property end sufficient for a fire truck; and vertical clearance of 4 metres for the full length.

Culverts crossing streams or drainage channels must be rated to carry a fire appliance weight (typically 25 tonnes gross).

For long driveways, passing bays every 100 metres allow appliances to pass in either direction.

These requirements are checked at planning consent and often at completion. A build with inadequate access can be refused approval or refused occupancy until access is remediated.

The proposed building must be positioned to meet minimum setbacks from classified vegetation, which are what drive the BAL rating.

Reducing the building distance from vegetation increases the BAL and construction cost. Increasing the distance reduces the BAL but requires more of the block for the building footprint.

On smaller bushfire-prone blocks, the design decision often becomes: reduce vegetation to bring more of the block into low-BAL zone, or accept higher BAL construction and retain more vegetation. Each has trade-offs.

Boundary setbacks to neighbours must also comply with standard planning rules regardless of bushfire considerations. Coordinate the two setback requirements at concept design.

Bushfire resilience is not a static state achieved at completion. The defendable space must be maintained year on year.

CFS bushfire preparation guidance is updated annually with recommended action lists. Key activities: cleaning gutters of leaf debris; mowing grass to specified heights; removing dead vegetation; pruning trees away from the building; storing firewood away from the house; maintaining the water supply and access.

Some council rules require vegetation management as an ongoing planning condition. Non-compliance can result in enforcement action and can affect insurance coverage.

Bushfire risk insurance often requires evidence of vegetation management as a condition of cover. Take annual photographs of the property in bushfire season to document ongoing compliance.

The CFS Bushfire Ready campaign runs annually and provides property owner support, including free property assessments in some areas. Take advantage of these services; they surface issues that owners often miss.