BAL ratings explained: design implications by bushfire band
What each BAL rating means for residential design in AU: BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40, BAL-FZ. Cost step-up + builder implications.
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The six BAL ratings (Bushfire Attack Levels) set out in AS 3959-2018 are not just numbers on a certificate. Each band imposes progressively tighter construction requirements that reshape your design, your material schedule, and your build cost. The step from BAL-LOW to BAL-12.5 is a modest premium. The step from BAL-29 to BAL-40 is where costs start to climb sharply. And BAL-FZ (Flame Zone) is the one that can kill a project entirely: the standard requires FRL-rated wall systems, no standard eave overhang, and minimum 10 m setback from vegetation. Many BAL-FZ sites are effectively non-buildable under standard residential construction, and no amount of material substitution fixes that.
Know your BAL before you draw. A BAL that goes up between pre-DA and DA lodgement due to vegetation changes can add tens of thousands to the cost estimate the client already signed off on.
What this article is for
When a site is in a Bushfire-Prone Area (BPA), the BAL drives a specific set of construction requirements under AS 3959-2018. This article covers what those requirements mean for design, what they cost at each band, and the practical traps builders and designers hit in the field.
The AS 3959-2018 standard itself is covered in AS 3959-2018 (compliance article). How BPAs are mapped by each state is covered in Bushfire-prone area mapping. This article sits between the two: it focuses on the design envelope implications of each BAL band, for builders, building designers, and architects trying to scope a job before the detail drawings start.
The six BAL ratings
All six levels are defined in AS 3959-2018. The ratings reflect the radiant heat flux a building may experience during a bushfire event, measured in kilowatts per square metre (kW/m²).
| BAL | Radiant heat | Threat type | Construction trigger |
|---|---|---|---|
| BAL-LOW | Below 12.5 kW/m² | Negligible | No special construction requirements |
| BAL-12.5 | Up to 12.5 kW/m² | Ember attack | Section 5 of AS 3959 |
| BAL-19 | 12.5 to 19 kW/m² | Ember attack + increasing heat | Section 6 of AS 3959 |
| BAL-29 | 19 to 29 kW/m² | Ember attack + significant heat | Section 7 of AS 3959 |
| BAL-40 | 29 to 40 kW/m² | Ember attack + heat + possible flame contact | Section 8 of AS 3959 |
| BAL-FZ | Above 40 kW/m² | Direct flame contact | Section 9 of AS 3959 |
Key construction requirements by band
The table below summarises the design-relevant requirements for a Class 1a dwelling. Full material specifications are in the standard.
| Element | BAL-12.5 | BAL-19 | BAL-29 | BAL-40 | BAL-FZ |
|---|---|---|---|---|---|
| Glazing (low level) | 4 mm toughened | 5 mm toughened | 5 mm toughened (windows), 6 mm toughened (doors) | 6 mm toughened, metal frames | FRL —/30/— or tested system |
| Wall cladding (below 400 mm) | Non-combustible or min. 6 mm cement sheet | Non-combustible or min. 6 mm cement sheet | Non-combustible, min. 6 mm cement sheet, or steel | Non-combustible, min. 9 mm cement sheet, or steel | Non-combustible or FRL 30/30/30 or tested system |
| Eave lining | 2 mm ember mesh to gaps | 2 mm ember mesh to gaps | Min. 4.5 mm cement sheet eave lining + ember mesh | Min. 6 mm cement sheet eave lining + ember mesh | FRL —/30/30 or tested system |
| Roof sarking | Required | Required | Required | Required | Required, non-combustible |
| Ember guards (vents, gutters) | Required (2 mm mesh) | Required (2 mm mesh) | Required (2 mm mesh) | Required (2 mm mesh, no aluminium) | Required |
| Evaporative coolers | Permitted | Permitted | Permitted | Not permitted | Not permitted |
| Eave overhang | Permitted | Permitted | Permitted | Permitted with screening | Effectively prohibited (FRL cladding systems required) |
| Subfloor (unenclosed) | Non-combustible or bushfire-resisting timber | Non-combustible or bushfire-resisting timber | Non-combustible or bushfire-resisting timber | Non-combustible or tested system | FRL 30/—/— + non-combustible, or tested system |
| Setback from vegetation | None mandated | None mandated | None mandated | None mandated | Minimum 10 m |
Notes:
- “Low level” glazing means glazing within 400 mm of the finished ground level, deck surface, or a roof less than 18 degrees.
- From BAL-40 upward, aluminium mesh is prohibited for screening. Steel or bronze only.
- Decking at BAL-12.5: no requirements if the deck is more than 300 mm from low-level glazing.
- At BAL-FZ, wall systems must achieve FRL 30/30/30 or comply with AS 1530.8.2 when tested from the outside.
Cost step-up per band
These figures are indicative. They reflect typical Class 1a dwellings and vary substantially based on construction type, site access, and current material pricing. Lightweight elevated construction (high-set, timber frame on stumps) carries the biggest premium at upper BAL bands because every element of the exposed underfloor structure is in scope.
| BAL | Typical premium over BAL-LOW |
|---|---|
| BAL-LOW | Base |
| BAL-12.5 | +$10,000 to $20,000 |
| BAL-19 | +$15,000 to $30,000 |
| BAL-29 | +$30,000 to $60,000 |
| BAL-40 | +$60,000 to $120,000 |
| BAL-FZ | +$150,000+ or effectively non-buildable under standard residential |
These ranges are consistent with adjusted ABCB Regulatory Impact Statement cost data and industry reports from bushfire construction specialists (verified 2026). They are not quotes. Brief your client that the actual premium depends on the specific design and the builder quoting it, and that BAL-40 and BAL-FZ are the bands where preliminary cost estimates most often understate the final figure.
A slab-on-ground, brick veneer house at BAL-12.5 sits at the lower end of the range because the base construction already meets most requirements. A lightweight timber-frame elevated house at BAL-40 sits well above the upper end.
How BAL is assessed
AS 3959-2018 sets out two methods.
Method 1 (simplified) uses look-up tables in the standard. The assessor identifies the vegetation classification for the site, the slope of the land, and the distance from the building to the vegetation. These three inputs produce a BAL from the table. Method 1 is the standard approach for most residential DA and building permit applications. An accredited Level 1 BAL assessor (or a bushfire consultant registered with the Fire Protection Association Australia, FPAA) can complete it.
Method 2 (detailed) involves a site-specific radiant heat flux calculation. It requires a qualified bushfire consultant and is used when Method 1 produces a result the consultant believes overstates the actual risk (for example, because of a topographic feature that interrupts line-of-sight to the vegetation, or because the vegetation classification is borderline). Method 2 can produce a lower BAL than Method 1 for the same site. The consultant takes professional responsibility for the result.
The BAL applies to the proposed building on the site at the time of DA lodgement or building permit application. It is not a permanent rating attached to the land. If the vegetation changes between assessment and consent, or between consent and build, the BAL may change.
Where BAL applies
BAL requirements apply only when a site is within a mapped Bushfire-Prone Area (BPA) under state legislation. Each state maps and labels these areas differently.
| State | Mapping mechanism | BAL trigger |
|---|---|---|
| NSW | Bush Fire Prone Land (BFPL) Map, certified by the NSW RFS Commissioner; checked via the NSW RFS online tool | Any development on BFPL must comply with Planning for Bush Fire Protection 2019; BAL assessment required as part of DA/CDC |
| VIC | Bushfire Management Overlay (BMO, Clause 44.06) and Bushfire-Prone Area (BPA) layer in planning schemes; Clause 53.02 sets bushfire planning objectives | Planning permit triggers a BAL requirement; building permit in a BPA requires a BAL certificate regardless of planning permit status. Note: Clause 53.02 was updated by VC248 in May 2026 |
| QLD | Bushfire overlay in each council’s planning scheme, based on state hazard mapping | Overlay triggers a Bushfire Management Plan as part of DA; BAL required for building permit in bushfire overlay areas |
| WA | Map of Bushfire Prone Areas designated under s.18P of the Fire and Emergency Services Act 1998; reviewed regularly by DFES; current version effective 13 December 2025 | All new habitable buildings in a BPA require a BAL assessment; BAL-40 or BAL-FZ on lots over 1,100 m² also requires Development Approval in addition to a building permit |
| SA | Defined Flood and Bushfire Overlay in the Planning and Design Code | BAL assessment required as part of the development application |
| TAS | Bushfire-prone area maps under the Tasmanian Planning Scheme | BAL assessment required in affected zones |
| ACT | Bushfire-prone area designation in the Territory Plan | BAL assessment required before building permit |
For NSW, check the NSW RFS Bush Fire Prone Land map before any site assessment. For VIC, check the BMO layer and BPA layer in VicPlan. For WA, check the DFES Bushfire Prone Areas map.
More on how each state maps BPAs is at Bushfire-prone area mapping.
What can go wrong
Assuming the existing house’s BAL applies to the new build. It doesn’t. The BAL is assessed for the proposed building at the time of application. If vegetation on or around the site has changed since the existing house was built, particularly if it has grown denser, the new build may land in a higher band than the old one.
The BAL-FZ trap. BAL-FZ requirements are fundamentally different in kind, not just degree. FRL-rated wall systems, glazing with FRL —/30/—, FRL-rated subfloor structure, minimum 10 m setback from vegetation. For a typical suburban or peri-urban site with limited lot depth, the 10 m setback alone can make the project non-compliant with the lot’s zoning setbacks, or eliminate any useable building envelope. Some councils and certifiers will tell you the lot is non-buildable at BAL-FZ. Always scope this before the client buys the land.
Vegetation change after consent. If vegetation is cleared or planted between DA consent and the building permit, the BAL can change. Councils and certifiers can require a fresh assessment at building permit stage even if one was done at DA stage.
Cost surprise on lightweight elevated construction. Builders familiar with slab-on-ground houses often underestimate the BAL-29 and BAL-40 premium on elevated lightweight frames. The entire exposed subfloor structure, bearers, joists, stumps, is in scope. At BAL-40, non-combustible or tested-system requirements for the subfloor can add more cost than all the glazing and cladding upgrades combined on a high-set home.
Fly screens and mesh. Standard aluminium fly screens are not compliant at BAL-29 and above for ember mesh applications. The standard requires 2 mm corrosion-resistant steel, bronze, or aluminium at BAL-29, but restricts this to steel or bronze (no aluminium) at BAL-40 and above. Many standard window and door schedules get this wrong on the first pass.
How to use this with related articles
- Bushfire-prone area mapping: check whether your site is in a BPA before commissioning a BAL assessment.
- Planning instrument hierarchy: where bushfire overlays and planning scheme controls sit in the overall hierarchy.
- SEPPs NSW and DCPs: the planning layer above the building standard layer; both can impose additional bushfire controls beyond the BAL.
- AS 3959-2018 (compliance): the full standard, section by section.
- NCC bushfire BAL requirements: how the NCC Volume Two Part 3.7.4 references AS 3959.
References
- AS 3959-2018 Construction of buildings in bushfire-prone areas, Standards Australia. See Standards Australia catalogue (paywalled; verified via secondary sources 2026-05-23).
- NSW Rural Fire Service, Planning for Bush Fire Protection 2019, via rfs.nsw.gov.au (verified 2026-05-23).
- NSW Rural Fire Service, Bush Fire Prone Land, via rfs.nsw.gov.au (verified 2026-05-23).
- Victoria Planning Provisions Clause 53.02 Bushfire Planning (updated by VC248, May 2026), via planning.vic.gov.au (verified 2026-05-23).
- CFA (Country Fire Authority), Planning and Bushfire Management Overlay, via cfa.vic.gov.au (verified 2026-05-23).
- DFES WA, Bushfire Prone Areas (current version effective 13 December 2025), via dfes.wa.gov.au (verified 2026-05-23).
- WA State Planning Policy 3.7: Bushfire, via planning.wa.gov.au (verified 2026-05-23).
- Bushfire Design Consultants, BAL construction requirement guides (BAL-12.5 through BAL-FZ), via bushfiredesignconsultants.com.au (verified 2026-05-23).
- ABCB Regulatory Impact Statement (2009) construction cost data, adjusted to current equivalents, via bushfireconsultantsnewcastle.com.au (verified 2026-05-23).
Related
- Bushfire-prone area mapping: how each state maps and triggers BPA controls
- SEPPs in NSW: what they are and when they affect your project
- DCPs: how to read development control plans for a residential project
- Australian planning scheme structure: how the regimes compare across all 8 states and territories
See also
- Submitting a DA in NSW, step-by-step
- Getting a planning permit in VIC, step-by-step
- AS 3959-2018 Construction of buildings in bushfire-prone areas (compliance)
- NCC bushfire BAL requirements
- NCC bushfire construction requirements
Last updated: 2026-05-23. Verified: 2026-05-23. Quarterly review for currency. Construction requirement specifics verified against AS 3959-2018 via Bushfire Design Consultants reference guides and ABCB sources. State BPA mapping links verified against each state’s primary planning or emergency services portal on 2026-05-23.