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Bushfire attack levels and prefab homes
| Governing standard | AS 3959:2018, referenced by the NCC[1][2] |
|---|---|
| Number of levels | Six — LOW, 12.5, 19, 29, 40, FZ[2] |
| What the number means | Radiant heat flux threshold in kW/m²[3] |
| BAL-12.5 | Ember attack, radiant heat to 12.5 kW/m²[2] |
| BAL-29 | Radiant heat 19–29 kW/m²[2] |
| BAL-FZ | Direct flame contact, radiant heat above 40 kW/m²[2] |
| Assessment cost | ≈ A$400 – $800 (assessor); A$800 – $2,500 (consultant)[4] |
| Published cost impact, BAL-FZ | A$50,000 to A$277,000 — sources disagree sharply[5][6] |
| *Cost figures on this page come from commercial and industry sources that disagree substantially. See § Cost. | |
A home on bushfire-prone land, factory-built or not, must meet AS 3959:2018, which the National Construction Code references; it sets six bushfire attack levels (BAL-LOW, 12.5, 19, 29, 40 and FZ), each tied to a radiant heat threshold in kW/m².[1][2][3] A BAL assessment costs about A$400–A$800 from an accredited assessor, and published estimates of the extra build cost at BAL-FZ range from over A$50,000 to A$277,000, with sources disagreeing sharply.[4][5][6]
A large share of Australia is mapped as bushfire-prone. Where a block falls inside that mapping, a bushfire attack level — universally shortened to BAL — is assigned to the proposed building, and that single classification then governs what the building may be made of.
The BAL is not a planning opinion. It is a calculated exposure figure derived from vegetation type, slope and separation distance, and the number attached to it is a radiant heat flux threshold measured in kilowatts per square metre.[3] The construction rules that follow are set by AS 3959, referenced by the National Construction Code as a Deemed-to-Satisfy solution.[1][2]
For prefab buyers this matters more than it does for conventional buyers, for a reason set out below: at the upper levels, a substantial part of the prefab and kit market simply cannot supply a compliant building.
The six levels §
| Level | Radiant heat | Principal exposure | AS 3959 section |
|---|---|---|---|
| BAL-LOW | — | Insignificant; no specific construction requirements | —[2] |
| BAL-12.5 | to 12.5 kW/m² | Ember attack | Sections 3 & 5[7] |
| BAL-19 | 12.5 – 19 kW/m² | Increasing ember attack and windborne debris | Sections 3 & 6[7] |
| BAL-29 | 19 – 29 kW/m² | Significant ember attack and burning debris | Sections 3 & 7[7] |
| BAL-40 | 29 – 40 kW/m² | Radiant heat with flame contact likely | Sections 3 & 8[7] |
| BAL-FZ | above 40 kW/m² | Direct flame contact | Sections 3 & 9[7] |
Every building on bushfire-prone land must meet the general construction requirements in Section 3 of AS 3959:2018 regardless of level; the BAL determines which additional section applies on top.[7]
The requirements escalate in a predictable direction. At BAL-12.5 the concern is embers: screening material is typically mesh or perforated sheet with a maximum 2 mm aperture, and joints, gaps or weepholes that will pass a 3 mm probe must be screened or sealed.[2] By BAL-29 and BAL-40, commercial guidance reports that steel, brick or concrete construction is increasingly specified and timber construction becomes very limited. At BAL-FZ, construction is typically non-combustible throughout, with no eaves or overhangs and very high-specification glazing.[8]
Individual elements carry their own requirements. Where sarking is required, for example, it must have a flammability index of not more than 5 when tested to AS 1530.2.[3]
How a level is assigned §
The methodology is set at state level. In New South Wales the statutory document is the NSW Rural Fire Service's Planning for Bush Fire Protection 2019, which sets out standards, specifications and requirements for development on bushfire-prone land; Appendix 1 contains the site-based assessment methodology and the tables used to calculate a BAL.[9][10]
Two points are commonly missed. First, the assessment covers more than the house: detached structures such as sheds and decks within 6 m of a building on bushfire-prone land must also meet the requirements.[10] Second, the assessment is bound up with the approval pathway. In NSW the Codes SEPP permits complying development on low bushfire risk land and requires a council or accredited bushfire consultant to provide a BAL certificate; the critical function of that certificate is to establish that the development has not been assessed at the higher levels.[9] Land zoned as at risk of ember attack or of exposure to flames from a fire front cannot use the complying development pathway at all.[11]
Commercial guidance reports assessment costs of about A$400 to $800 for a formal assessment by an accredited assessor, and A$800 to $2,500 for a consultant who also advises on reducing the BAL through vegetation management and building placement.[4]
What the code does at the top of the range §
BAL-FZ is not simply an expensive version of BAL-40. It is treated differently in the code itself.
In New South Wales, buildings subject to BAL-FZ must comply with specific conditions of development consent for construction at that level, or with requirements modified by development consent following consultation with the NSW RFS under section 4.14 of the Environmental Planning and Assessment Act 1979, or with a bushfire safety authority under section 100B of the Rural Fires Act 1997.[1] In Victoria, certain buildings subject to a BAL exceeding BAL-12.5 have no Deemed-to-Satisfy Provisions at all and require a Performance Solution.[1]
The practical translation is that above a certain point there is no off-the-shelf compliance path. Commercial guidance reports that some councils do not permit new homes in BAL-FZ areas.[4]
Cost — where the published figures disagree §
This is the part of the topic where sources conflict most sharply, and the disagreement is worth setting out rather than resolving.
| Source | BAL-12.5 | BAL-29 / 40 | BAL-FZ |
|---|---|---|---|
| PropDex, 2026[8] | 8,000 – 15,000 | 30,000 – 60,000 | 80,000+ |
| HomeBuild AI, 2026[4] | — | 40,000 – 80,000 (BAL-40) | 80,000 – 200,000+ |
| Insurer estimate reported 2018[5] | 16,000 | 56,000 (BAL-40) | 65,000 – 277,000 |
| BAL Assessments[6] | — | — | above 50,000 |
| Fire Australia / regional WA study[12] | — | under 10% of build cost at BAL-40 | significant increase |
The last row is the one that does not fit the others. That study used a building design common in regional Western Australia rather than a hypothetical plan, and found that building to BAL-40 added less than 10 per cent to the cost of the reference house, with significant increases appearing only at flame zone.[12] On a A$500,000 house that is under A$50,000 — within range of the commercial figures — but on a smaller dwelling it is a far lower number than any of them.
A bushfire consultant reviewing the same question observes directly that there has not been much research on the construction-cost difference between BAL levels, despite construction standards being treated as one of the major bushfire protection measures.[5] No independent Australian survey of BAL construction costs was found among the sources compiled here. The figures above should be read as indicative order-of-magnitude guidance, not as prices.
Where sources do agree is on which elements drive the cost: gap and join sealing, glazing upgrades, screens, bushfire shutters or BAL-40/FZ-rated window systems, and lining of eaves and verandas.[12]
Why this hits prefab buyers differently §
Two findings in the published material bear directly on off-site construction.
The first is a supply constraint. An assessor-published guide states plainly that many kit home suppliers and other builders cannot comply with BAL-FZ and BAL-40, and possibly BAL-29, and notes that only a limited number of flame zone roofing solutions have been accepted and that flame zone window solutions are only slowly becoming available.[6] A standard design offered from a catalogue is engineered to a particular specification; raising that specification is not always a variation the manufacturer can price.
The second is a materials bias. The regional WA study noted that compliance costs favour some construction types — brick veneer among them — and observed that this potentially reduces consumer choice and design innovation for alternative construction types.[12] Lightweight, panelised and timber-framed systems, which describe much of the Australian prefab sector, sit on the wrong side of that bias at the upper levels.
Neither finding means prefab cannot be built on bushfire-prone land. Several builders listed in Australian directories describe products as bushfire-compliant or BAL-rated. It means the BAL should be established before a design is selected, not after — because at BAL-29 and above the number can remove options from the list rather than simply add cost to them.
Beyond construction: insurance and lending §
Commercial guidance reports that a BAL rating affects insurance premiums and, in some cases, finance: that mainstream insurance becomes harder to obtain at BAL-40 and BAL-FZ, and that some lenders require specific assessment before providing finance for BAL-FZ properties.[8] These are commercial observations rather than published policy positions, and no insurer or lender documentation confirming them was found among the sources compiled here. Anyone relying on them should confirm directly with the insurer and the lender before committing to a block.
What this page does not cover §
State-by-state assessment methodologies outside New South Wales — including the Victorian Bushfire Management Overlay, and the equivalent instruments in Queensland, South Australia, Western Australia and Tasmania — are not covered by the sources compiled here. The construction requirements in AS 3959 apply nationally through the NCC, but the way a BAL is determined and the planning consequences that follow differ by jurisdiction.
The full construction requirements of AS 3959:2018 are not reproduced here. The standard is a copyright document available through Standards Australia and authorised distributors.
Sent to info@modularlivingaustralia.com.au and handled by a person. See Privacy.
References §
- Australian Building Codes Board — National Construction Code — Part G5 Construction in bushfire prone areas — compliance with AS 3959 achieves compliance with Part G5 for the relevant building classes. In New South Wales, buildings subject to BAL-FZ must comply with specific conditions of development consent for construction at that level, or with requirements modified by development consent following consultation with the NSW Rural Fire Service under section 4.14 of the Environmental Planning and Assessment Act 1979, or with a bushfire safety authority under section 100B of the Rural Fires Act 1997. In Victoria, certain buildings subject to a BAL exceeding BAL-12.5 have no Deemed-to-Satisfy Provisions and require a Performance Solution. ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-one/g-ancillary-provisions/part-g5-construction-bushfire-prone-areas
- Intertek Inform — AS 3959 explainer published by a standards distributor — AS 3959:2018 prescribes construction details to reduce the risk of ignition from bushfire, applying to sites assessed as BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40 or BAL-FZ. Radiant heat thresholds: BAL-12.5 up to 12.5 kW/m²; BAL-19 between 12.5 and 19; BAL-29 between 19 and 29; BAL-40 between 29 and 40; BAL-FZ above 40 with direct flame exposure. BAL-LOW carries no specific construction requirements. The standard is referenced in the NCC as a Deemed-To-Satisfy Solution and as an Acceptable Construction Manual in Volume Two. Ember screening typically requires mesh or perforated sheet with a maximum 2 mm aperture, and joints, gaps or weepholes passing a 3 mm probe must be screened or sealed. intertekinform.com/en-gb/resources/blog/as-3959--construction-of-buildings-in-bushfire-prone-areas-what-you-need-to-know/
- James Hardie — Construction of buildings in bushfire prone areas technical supplement — there are six bushfire attack levels and AS 3959:2018 contains construction solutions for all of them, with construction requirements increasing as the BAL rises; the number after "BAL" is based on heat flux exposure thresholds in kW/m². Where sarking is required it must have a flammability index of not more than 5 when tested to AS 1530.2. Manufacturer-published technical guidance. jameshardie.com.au/ContentfulCMS/Technical-Library/Bushfire_Prone_Area_Technical_supplement.pdf
- HomeBuild AI — BAL ratings explained, 2026 — a formal assessment by an accredited assessor costs about A$400–$800, and a bushfire consultant providing assessment plus advice on minimising the BAL about A$800–$2,500. Reports typical additional construction cost of A$40,000–$80,000 above standard construction for BAL-40 and A$80,000–$200,000 or more for BAL-FZ, and states that some councils do not allow new homes in BAL-FZ areas at all. Commercial guidance. homebuildai.com.au/blog/bal-ratings-explained
- Bushfire consultant, Newcastle (BEMC) — BAL construction costs — notes that little research exists on the construction-cost difference between BAL levels despite construction standards being viewed as a major bushfire protection measure. Reports a 2018 estimate attributed to a major insurer of A$16,000 for BAL-12.5 rising to A$56,000 for BAL-40, and A$65,000 to A$277,000 for flame zone construction, and references a 2009 Australian Building Codes Board Regulatory Impact Statement analysing three construction designs. Consultancy-published guidance. bushfireconsultantsnewcastle.com.au/bal-construction-costs/
- BAL Assessments — BAL and building costs — reports additional cost for BAL-FZ above A$50,000, notes that limited flame zone elements have been developed and approved, that only a limited number of flame zone roofing solutions have been accepted, and that flame zone window solutions are only slowly becoming available. States that many kit home suppliers and other builders cannot comply with BAL-FZ and BAL-40, and possibly BAL-29. Assessor-published commercial guidance. bal.net.au/cost.htm
- Bushfire Design Consultants — AS 3959:2018 reference guide — all buildings must meet the general construction requirements of Section 3 of AS 3959:2018, with a BAL assessment determining which of the additional sections applies: Section 5 for BAL-12.5, Section 6 for BAL-19, Section 7 for BAL-29, Section 8 for BAL-40 and Section 9 for BAL-FZ. Consultancy-published guidance. bushfiredesignconsultants.com.au/as-3959
- PropDex — Bushfire attack level ratings explainer — reports additional construction cost above BAL-LOW of A$8,000–$15,000 for BAL-12.5, A$30,000–$60,000 for BAL-29 and BAL-40, and A$80,000 and above for BAL-FZ. States that at BAL-29 and BAL-40 steel, brick or concrete construction is increasingly specified and timber construction becomes very limited; that at BAL-FZ construction is typically non-combustible throughout with no eaves or overhangs; and that some lenders require specific assessment before providing finance for BAL-FZ properties. Commercial guidance. propdextest.com/articles/bushfire-attack-level-bal-rating-construction-insurance
- NSW Rural Fire Service — Bush Fire Attack Level (BAL) Rating Fact Sheet, November 2021 — Planning for Bush Fire Protection 2019 describes a BAL as a means of measuring the severity of a building's potential exposure to ember attack, radiant heat and direct flame contact; Appendix 1 of PBP 2019 details the site-based assessment methodology and assessment tables. The Single Dwelling Application Kit can be used for BAL 29 and below, though councils reserve the right to accept or reject it. The Codes SEPP enables complying development on low bushfire risk land and requires a council or accredited bushfire consultant to provide a BAL certificate; the most important element of that certificate is the identification that the development is not assessed at the higher levels. rfs.nsw.gov.au/__data/assets/pdf_file/0015/234024/BAL-Rating-Fact-Sheet.pdf
- NSW Rural Fire Service — Building on bush fire prone land, additional information — Planning for Bush Fire Protection 2019 is a statutory document produced by the RFS setting out standards, specifications and requirements for development on bushfire-prone land. Detached structures such as sheds and decks within 6 m of a building on bushfire-prone land are also required to meet PBP. rfs.nsw.gov.au/plan-and-prepare/building-on-bush-fire-prone-land/additional-information
- NSW Department of Planning, Housing and Infrastructure — Secondary dwellings (granny flats) — additional requirements apply on bushfire-prone land, and land zoned as at risk of ember attack or of exposure to flames from a fire front cannot use the complying development pathway. planning.nsw.gov.au/the-planning-system/disaster-recovery/secondary-dwellings-granny-flats
- Biodiverse Solutions / Fire Australia project report — Cost of building homes that will survive bushfire — a study using a building design common in regional Western Australia rather than hypothetical plans found that building to BAL-40 added less than 10 per cent cost to the reference house, with significant cost increases occurring only at the BAL-FZ standard. Identifies gap and join sealing, glazing upgrades, screens, bushfire shutters or BAL-40/FZ-rated window systems, and lining of eaves and verandas as the largest cost drivers, and notes that costs favour some construction types such as brick veneer, potentially reducing consumer choice for alternative construction types. biodiversesolutions.com.au/wp-content/uploads/2014/05/Fire-Aust_Project-BAL-Build.pdf
Figures are as published by the sources above at their respective dates and are paraphrased summaries, not verbatim reproductions. Verify current pricing and rules directly with builders, lenders and planning authorities.