PERTHBuilding in a bushfire-prone area: what a BAL rating costs you
Bushfire Attack Level assessment under AS 3959: the bands from BAL-LOW to BAL-FZ, when it is required, what each level costs to build
If your block is shown as bushfire prone on the WA map, building on it costs more, takes an extra approval step, and follows a construction standard most buyers have never heard of. Here is what a Bushfire Attack Level assessment is, when you need one, and why it is cheaper to deal with at the planning stage than at any point after.
What a BAL is
A Bushfire Attack Level is a measure of how severely a building could be exposed to a bushfire — specifically to ember attack, radiant heat, and direct flame contact. It is expressed in increments of radiant heat in kilowatts per square metre, and it runs in bands: BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40, and BAL-FZ, the flame zone, which is the most severe.
The assessment is carried out under AS 3959, the Australian Standard for construction of buildings in bushfire-prone areas. It takes account of the vegetation around the site, the slope of the land, the distance from the building to the classified vegetation, and the fire weather of the region. From those factors it produces the BAL rating, and the rating determines the construction requirements.
When you need one
A BAL assessment is generally required when a new dwelling or habitable building is proposed on land designated bushfire-prone by the Department of Fire and Emergency Services, when significant additions or renovations are planned for an existing building in such an area, or when you apply for a building permit where a bushfire hazard has been identified. Even a modest development can trigger it if it sits close enough to classified vegetation.
You can check whether your block is affected on the WA Map of Bush Fire Prone Areas. Shown in pink? Then this applies to you.
What the rating actually costs you
The construction requirements escalate with the band. At the lower levels the concern is embers — sealing gaps, screening vents and weep holes, and using materials that will not ignite from windblown embers. At the higher levels the building has to withstand serious radiant heat and, at BAL-FZ, direct flame contact, which drives requirements for roofs, external walls, windows, shutters, doors, decks and underfloor areas, and can extend to a dedicated water supply for firefighting.
Attached structures come along for the ride. A garage, carport, deck or veranda attached or adjacent to the building generally has to meet the same requirements, or be adequately separated from it. That is a detail that catches owner-builders in particular.
A higher BAL raises the build cost, sometimes considerably, which is why getting a qualified assessor to find the most cost-effective compliant approach is worth doing early rather than accepting the first number.
Why the timing is everything
BAL requirements form part of the planning and building permit process. Addressing them at the planning stage, before design is locked, is straightforward. Discovering an omission or error after construction is well under way is expensive and slow, because bushfire construction elements are difficult to retrofit once the building is up.
One honest caveat that AS 3959 states itself: meeting the standard improves a building's performance in a bushfire, but no construction guarantees survival, because fire and extreme weather do not behave predictably.
Where an inspection fits
On a new build in a bushfire-prone area, whether the BAL requirements have actually been met is something we check during staged inspections and at practical completion. Roofing and external walls, penetrations and eaves, windows and doors, decks and underfloor areas — these are the elements where an ember gap gets missed, and where the error is far easier to fix before handover than after. Where something falls short, the report identifies the element and the requirement it fails, so it can be put right while it still can be.
What the rating actually controls
A bushfire attack level assessment grades the level of bushfire risk a building faces, from the lowest exposure up to direct flame contact, and that grading drives the construction requirements that apply to the building. The higher the assessed level, the more stringent, and the more expensive, the construction has to be: the materials of the external walls, the type of windows and their glazing, the screening of vents and gaps to keep embers out, the detailing of the roof and the decks and the subfloor. At the lower levels the requirements are modest; at the higher levels they become substantial, because the building is being asked to withstand radiant heat, ember attack, and in the worst case flame contact. The BAL is, in effect, the translation from "how exposed is this site" to "how the building has to be built to survive that exposure".
This is why the assessment is not a formality. It determines a real and sometimes large cost, and it does so before a brick is laid, because the requirements it triggers have to be designed into the building from the start. A difference of one level can mean a meaningful difference in the cost of the windows, the walls, and the detailing, which is why getting the assessment right, neither over-stating the exposure and building in unnecessary cost, nor under-stating it and building a house not adequate for its setting, matters both financially and for safety.
What drives the rating
The assessed level depends on the relationship between the building and the bushfire hazard around it, and a handful of factors do most of the work. The type of vegetation nearby matters, because different vegetation burns with different intensity: forest presents a greater hazard than grassland. The distance between the building and that vegetation matters, because radiant heat and ember density fall off with distance, so a building set well back from the bush faces a lower level than one with vegetation pressing against it. And the slope of the land matters, because fire behaves more aggressively running uphill, so a building above a downslope of vegetation faces a higher exposure than one on flat ground or above an upslope.
Because these factors combine, two houses in the same broad area can carry quite different BAL ratings depending on their specific siting, the vegetation immediately around them, and the lie of the land. This is also why managing the vegetation and the setting around a building can influence its exposure: the hazard is not fixed by the map alone but by the actual relationship between the building and the fuel around it, which is partly within an owner's control through how the surrounds are maintained.
Timing, and where it fits a purchase
The timing of a BAL assessment is driven by building and planning: it is required in connection with building work in a designated bushfire-prone area, and it needs to be done as part of the design and approval process, because it determines the construction standards the work has to meet. Leaving it late is expensive, because a higher-than-expected rating discovered after a design is well advanced can force costly changes, so on a build in a bushfire-prone area the assessment is something to get done early, when its findings can shape the design rather than disrupt it.
For someone buying an existing house in a bushfire-prone area, a BAL assessment is not part of a standard purchase and a building inspection is not a bushfire assessment; the two are separate pieces of work. But understanding whether a property is in a designated bushfire-prone area, and what that means for any future building or significant renovation, is part of buying well in these areas, because it bears on what you will be able to do with the property and at what cost. An inspection of the building can, as part of assessing its condition, note the ember-related condition factors, gutters, vent screening, deck and subfloor exposure, that bear on real bushfire vulnerability, but the formal BAL assessment for building purposes is specialist work commissioned when building or renovating rather than part of the purchase inspection.
Common questions
What is a BAL assessment?
A Bushfire Attack Level assessment measures a building's potential exposure to ember attack, radiant heat and direct flame contact, under AS 3959. It produces a rating from BAL-LOW to BAL-FZ that sets the construction requirements.
When do I need one in WA?
Generally for a new dwelling, or significant additions or renovations, on land designated bushfire-prone by DFES, or when applying for a building permit where a bushfire hazard is identified.
Does a higher BAL cost more to build?
Yes, and the increase can be considerable at the higher bands. Addressing it at the planning stage is far cheaper than retrofitting bushfire construction after the build.
This comes up on every building stage inspections and practical completion (pci) inspection we carry out.
Read next: metal roofs act like a wing. Tie-downs are what stop them leaving. and buying a house backing onto bush: what to know first.
Sources
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