Understanding BAL ratings in South Australia

The BAL scale under AS 3959-2018 defines six levels of bushfire attack severity.

BAL-Low: insufficient risk to require specific bushfire construction measures. No AS 3959 provisions apply.

BAL-12.5: ember attack is the primary threat. Construction requirements focus on window screens, ember-proof roof detailing, and non-combustible materials in specific vulnerable areas.

BAL-19: ember attack plus low radiant heat (up to 19 kW/m²). Requirements expand to include some wall and roof cladding restrictions.

BAL-29: ember attack plus significant radiant heat (up to 29 kW/m²). Bushfire-resistant materials required for walls, roof, windows and doors.

BAL-40: severe radiant heat (up to 40 kW/m²) plus embers. Very restrictive material requirements; construction cost increases materially.

BAL-FZ (Flame Zone): direct flame contact possible. Highest level of protection required, with steel-framed construction, radiant heat barriers, and severely restricted material choices.

BAL determination is a technical assessment done by an accredited Bushfire Attack Level assessor. The assessment considers four factors.

First, the Fire Danger Index applicable to your area. This is set by state legislation and reflects the historical fire weather risk. In South Australia the index varies by region; the Adelaide Hills and Fleurieu are typically high, coastal plains lower.

Second, the classification of surrounding vegetation. Vegetation is classified into groups (forest, woodland, shrubland, grassland, or none) based on the type and density of plant cover within a defined distance of the proposed building.

Third, the distance from the classified vegetation to the proposed building. Closer vegetation increases the BAL.

Fourth, the slope of the land between the vegetation and the building. Steeper upslopes to the vegetation source increase the BAL because fire runs uphill faster and hotter.

The assessor combines these factors using AS 3959 methodology and produces a BAL certificate that becomes part of the development application.

Any block in a South Australian bushfire-prone area requires a BAL assessment before development approval. The assessment is commissioned by the property owner or their designer, typically at concept design stage.

A BAL assessor visits the site, records the vegetation classification, distance and slope, and produces a written report. Fees typically range from a few hundred to just over a thousand dollars depending on site complexity.

For a property being purchased with a view to building, a pre-purchase BAL assessment is worth commissioning. A BAL-FZ property will have dramatically different build economics compared to a BAL-19, and this can change the purchase decision.

Some council mapping is available online showing indicative BAL risk. This is a screening tool only; the formal assessment is site-specific and often produces a different result from the mapping.

The BAL of a property is not fixed. Removing or thinning classified vegetation within the assessment distance can reduce the BAL and materially reduce construction requirements.

This creates a design decision: retain vegetation for landscape amenity at higher construction cost, or manage vegetation to reduce BAL and construction cost.

Vegetation management on your own property is generally within your rights, subject to any council tree protection rules and any biodiversity protection zones. Removing vegetation on adjoining or public land is usually not permitted.

For blocks close to a boundary with a bush reserve or national park, the vegetation classification typically cannot be managed away. These blocks often have persistent high BAL ratings regardless of on-property landscaping.

The cost premium of building to BAL requirements increases significantly at each level. Rough guidance: BAL-12.5 adds a modest few percent to build cost. BAL-19 adds several percent. BAL-29 adds around 10 percent. BAL-40 can add 15 to 20 percent. BAL-FZ can add 25 to 40 percent or more.

These numbers depend on the specific design and material choices. Some designs handle BAL requirements more cost-effectively than others. Steel-framed construction becomes attractive at BAL-40 and above. Timber cladding becomes prohibitive at BAL-29 and above.

For a block with BAL-40 or BAL-FZ rating, engaging a builder experienced with high BAL construction is essential. The specialist pool is small in South Australia, mainly active in the Adelaide Hills. Volume builders often decline BAL-40 and BAL-FZ builds entirely.