Granny Flat in a Bushfire Zone
A secondary dwelling on a bushfire-prone block follows the same Australian Standard as the main house – AS 3959 Construction of buildings in bushfire-prone areas. The BAL rating, the approval pathway and the construction cost all change depending on where you build, what vegetation surrounds the site, and which state you are in. This guide covers the full process from assessment through to construction, state by state.
Why Bushfire Rules Apply to Secondary Dwellings
A granny flat, secondary dwelling or ancillary accommodation is a Class 1a building under the National Construction Code. The NCC’s Part G5 (formerly 3.7.4 in the Housing Provisions) requires every Class 1a building on land designated as bushfire-prone to be constructed to the Bushfire Attack Level determined by an AS 3959 assessment. There is no exemption for size. A 40-square-metre studio triggers the same BAL compliance as a 300-square-metre house on the same block.
The designation itself comes from state mapping. Every state and territory publishes a bushfire-prone area map – maintained by the Rural Fire Service in NSW, the Country Fire Authority in Victoria, the Department of Fire and Emergency Services in WA, and their equivalents elsewhere. If your block falls inside a designated area, the BAL assessment is mandatory before any building permit issues.
The key point for secondary dwellings. The granny flat gets its own BAL rating, assessed at its own building footprint. Moving the footprint 15 metres further from the tree line can drop the rating by one or two levels – and save tens of thousands of dollars in construction cost. The main house’s rating does not automatically transfer to the secondary dwelling.
What a BAL Assessment Measures
A BAL assessment evaluates four factors at the proposed building location.
Vegetation type and classification. AS 3959 classifies vegetation into seven groups – forest, woodland, shrubland, scrub, mallee/mulga, rainforest and grassland – each carrying a different heat load in a fire. A block backing onto tall eucalypt forest generates a higher rating than one adjoining managed grassland, even at the same distance.
Distance from classified vegetation to the building. Measured horizontally from the outer wall of the proposed dwelling to the nearest edge of classified vegetation. Every additional metre of separation lowers the potential radiant heat exposure. For secondary dwellings, this measurement is taken from the granny flat’s walls, not the main house.
Slope of the land. Fire travels faster uphill. A downslope from vegetation to building reduces the rating; an upslope increases it. AS 3959 applies slope adjustments in defined bands – flat/downslope, upslope to 5 degrees, 5 to 10 degrees, 10 to 15 degrees, and 15 to 20 degrees.
Fire Danger Index (FDI). A fixed value determined by the geographic region, not the individual property. Each state’s fire weather districts carry a designated FDI that reflects historical fire weather conditions. The FDI is looked up, not measured on site.
The output is a single BAL rating for the building footprint. The rating drives everything that follows – construction standards, material selection, glazing specifications, and cost.
The Six BAL Ratings
BAL-LOW. The site is at insufficient risk to warrant specific bushfire construction measures under AS 3959. Standard NCC construction applies. This rating is uncommon in designated bushfire-prone areas – most sites rate BAL-12.5 or above once classified vegetation is within 100 metres.
BAL-12.5. Ember attack is the primary threat. The building envelope must be sealed against ember entry – metal mesh screens with a maximum 2 mm aperture on all vents and openings, non-combustible guttering, and ember guards on weep holes and gaps. Glazing must be minimum 4 mm toughened glass. External walls can be timber-framed with compliant cladding.
BAL-19. Increased ember attack with rising radiant heat exposure. External walls require non-combustible cladding or tested systems. Windows need minimum 5 mm toughened glass. Doors must be solid-core timber (minimum 35 mm) or non-combustible with metal mesh screens. Timber decking requires specific species or fire-retardant treatment.
BAL-29. High radiant heat in addition to ember attack. All external cladding must be non-combustible. Subfloor areas must be enclosed. Timber decking is restricted to hardwood species tested to the standard, or non-combustible alternatives. Window openings require bushfire-rated shutters or screens plus toughened glass.
BAL-40. Very high radiant heat with probable flame contact. Full non-combustible external construction is required – no timber cladding, no timber framing exposed externally. Glazing must be bushfire-rated (typically toughened safety glass in steel or aluminium frames with specific seals). All gaps, joints and penetrations in the building envelope must be sealed against radiant heat and ember ingress.
BAL-FZ (Flame Zone). Direct flame contact and extreme radiant heat. Every external element must be individually tested and certified to AS 1530.8.2 or equivalent. This is the most restrictive and most expensive rating. In NSW, the Rural Fire Service will generally not support new secondary dwellings in flame zones. Other states require specific engineering solutions and may impose conditions on siting, water supply and access.
What a Higher BAL Rating Adds to the Build
The cost of BAL compliance is not the assessment itself but the construction upgrades the rating demands. For a secondary dwelling of 50 to 80 square metres, the typical additional cost as a percentage of the base build is roughly as follows.
BAL-LOW. No additional cost. Standard construction.
BAL-12.5. A 2 to 5 per cent premium. Primarily ember-sealing measures – mesh screens, non-combustible guttering, sealed gaps.
BAL-19. A 4 to 10 per cent premium. Non-combustible external cladding and upgraded glazing become the main cost drivers.
BAL-29. An 8 to 17 per cent premium. The full non-combustible external envelope, enclosed subfloors, upgraded doors, and restricted decking materials push costs higher.
BAL-40. A 17 to 33 per cent premium. Bushfire-rated glazing, fully non-combustible construction, and sealed building envelope.
BAL-FZ. A premium exceeding 35 per cent. Individually tested and certified elements, specialist engineering, and restricted material choices. Some suppliers will not quote for flame-zone secondary dwellings at all.
These percentages are indicative and vary with design, materials and site conditions. The percentage premium is proportionally higher for secondary dwellings than for larger homes because the base cost is lower while many of the compliance measures cost the same regardless of building size.
Planning a build in a bushfire-prone area?
iBuild supplies kit homes rated to every BAL level, from BAL‑LOW through to BAL‑FZ. Talk to our team about your site requirements and get a tailored quote.
State-by-State Approval Pathways
Each state handles secondary dwelling approvals differently, and bushfire-prone land adds an overlay to every pathway. The approval route, the referral body, and the BAL threshold that triggers a more complex process all vary.
New South Wales
Size limit. 60 square metres gross floor area. Minimum lot size 450 square metres.
Standard pathway. A Complying Development Certificate (CDC) through a private certifier offers a fast-track route – typically 10 to 20 business days. CDCs for secondary dwellings are assessed under the State Environmental Planning Policy (Affordable Rental Housing).
Bushfire overlay. Properties at BAL-LOW through BAL-29 can still use the CDC pathway, provided a Bush Fire Attack Level Certificate (formerly a section 4.14 certificate or BAL assessment) confirms the rating. Properties at BAL-40 or BAL-FZ cannot use complying development – they require a full Development Application (DA) through council, which includes referral to the NSW Rural Fire Service. The RFS assesses the proposal against Planning for Bush Fire Protection 2019 and may impose conditions on siting, access, water supply and landscaping – or refuse to support the application.
Key fact. The NSW RFS has a general position of not supporting new secondary dwellings in flame zones (BAL-FZ). This is not a blanket prohibition in legislation, but a policy position that makes BAL-FZ granny flat approvals in NSW exceptionally difficult.
Victoria
Recent change – Victorian size limit increased
Victoria has increased the maximum floor area for small second dwellings from 60 to 80 square metres, excluding decks and pergolas. This change significantly expands the range of granny flat designs available for bushfire-prone blocks in Victoria.
Size limit. 80 square metres floor area under the small second dwelling provisions, excluding decks and pergolas.
Standard pathway. A building permit is always required. A planning permit is generally not required for a small second dwelling in most residential zones, thanks to the VC253 amendment (effective December 2023) which streamlined the pathway.
Bushfire overlay. If the property sits within a Bushfire-Prone Area (BPA), a BAL assessment is required for the building permit. If the property also falls within a Bushfire Management Overlay (BMO), a planning permit is required and the application is referred to the Country Fire Authority. The CFA assesses against Clause 13.02 of the Victoria Planning Provisions and may require a Bushfire Management Plan covering water supply, access, vegetation management and defendable space.
Key fact. Victoria has two overlapping systems – the BPA (building-level) and the BMO (planning-level). A property can be in a BPA without being in a BMO, in which case only the BAL construction requirements apply. A property in a BMO triggers both the planning referral to CFA and the BAL construction standards.
Queensland
Size limit. Varies by council. Brisbane allows up to 80 square metres, Gold Coast up to 90 square metres in rural zones, Sunshine Coast 60 square metres, and Moreton Bay up to 100 square metres on rural or rural residential land.
Standard pathway. Building approval through a licensed private building certifier is always required. Development approval through council may or may not be needed – properties that meet the council’s “accepted development” criteria can proceed without it.
Bushfire overlay. Properties identified in the state’s bushfire hazard mapping require a BAL assessment as part of the building approval. Where the bushfire hazard overlay triggers code assessment or impact assessment under the local planning scheme, a development application with bushfire-specific supporting information is required. The Queensland Fire and Emergency Services (QFES) may be a referral agency.
Key fact. Since September 2022, Queensland has removed occupancy restrictions on secondary dwellings. A granny flat can be rented to anyone on the open market, not just family members – which makes the investment case for BAL-compliant construction stronger.
South Australia
Size limit. Generally 60 square metres or 70 per cent of the main dwelling’s footprint, whichever is smaller. Called “ancillary accommodation” under the Planning and Design Code.
Standard pathway. Development consent application through the PlanSA portal or local council. Assessed as a “performance assessed” development in most zones.
Bushfire overlay. Properties in designated bushfire-prone areas trigger an automatic referral to the Country Fire Service (CFS) Development Assessment Service. The CFS has 30 business days to assess and respond. The assessment covers siting, construction standards, access for emergency vehicles, vegetation clearance, and water supply. South Australia applies the Minister’s Specification SA 78 for additional construction requirements beyond AS 3959 in high-risk areas.
Key fact. SA’s bushfire mapping was updated in a state-wide code amendment process (2024-2025). Check the current mapping on the SA Planning Portal rather than relying on older information – some properties have been added to or removed from designated areas.
Tasmania
Size limit. 60 square metres or 30 per cent of the main dwelling’s gross floor area, whichever is less. A reform to increase the maximum to 90 square metres was initiated in March 2026 and is expected to take effect in late 2026 after public consultation.
Standard pathway. A planning permit is generally not required in most residential zones if the dwelling meets the Acceptable Solutions. A building permit is always mandatory.
Bushfire overlay. Properties in designated bushfire-prone areas require a BAL assessment as part of the building permit application. Tasmania’s Building Amendment (Bushfire-prone Areas) Regulations align with AS 3959 and the NCC’s Part G5. The Tasmania Fire Service may be consulted on applications in high-risk zones.
Key fact. Large portions of Tasmania’s urban fringe are designated bushfire-prone. The state’s building regulations require BAL compliance for all new Class 1a buildings in these areas, with no exemption for secondary dwellings regardless of size.
Western Australia
Size limit. Generally 70 square metres gross floor area under the Residential Design Codes.
Standard pathway. A building permit from local government is required. A planning permit may also be required if the proposal does not meet the R-Codes deemed-to-comply provisions.
Bushfire overlay. Properties in designated bushfire-prone areas require a BAL assessment by an accredited assessor (accredited through the Fire Protection Association Australia). Construction must comply with both AS 3959 and the Building Code of Australia. The Department of Fire and Emergency Services (DFES) maintains the Map of Bushfire-Prone Areas. Newly designated areas have a four-month transition period before construction requirements apply.
Key fact. In WA, BAL assessments are valid for only 12 months and must be reassessed if conditions change or the assessment expires before a building permit is granted. Vegetation regrowth, neighbouring development, or updated mapping can change the rating between assessments.
Siting Decisions That Affect the Rating
The BAL rating is calculated at the building footprint, not at the property boundary. This gives secondary dwellings a significant planning advantage over the main house – because the granny flat is smaller and more flexible to position, you can often site it to achieve a lower BAL rating than the main dwelling carries.
Maximise distance from classified vegetation. Every additional metre between the granny flat and the nearest classified vegetation reduces radiant heat exposure. Moving a 60-square-metre building footprint 10 metres further from the tree line can drop the rating from BAL-29 to BAL-19 – a saving of $6,000 to $10,000 in construction cost.
Use the main house as a radiation shield. AS 3959 allows for shielding – where an existing non-combustible building between the vegetation and the proposed dwelling reduces the radiant heat reaching the new structure. Siting the granny flat so the main house sits between it and the primary fire direction can reduce the BAL rating on the sheltered side.
Work with the slope. Fire travels faster uphill. If the block slopes, siting the granny flat on the downslope side of the vegetation (where the vegetation is above the building) attracts a higher rating than siting it on level ground or where the building is above the vegetation. Where possible, choose a location where the effective slope from vegetation to the building is flat or downhill.
Manage vegetation within the defendable space. Most bushfire planning frameworks require an Asset Protection Zone (APZ) or defendable space around a dwelling. For a secondary dwelling, this is measured from the granny flat’s walls. Reducing vegetation density and height within this zone – by removing understorey, reducing canopy cover, and maintaining low groundcover – can support a lower BAL rating at the building footprint.
Consider the access route. Emergency vehicle access to the secondary dwelling is assessed in most states. A granny flat at the rear of a deep block may require a dedicated access path meeting minimum width (typically 4 metres) and load-bearing requirements for fire trucks.
Commission the BAL assessment before finalising the design. The assessment is site-specific and footprint-specific. A $500 to $2,000 BAL assessment done early in the process – before plans are drawn – lets you test different siting options and choose the footprint location that minimises the rating. Moving the building after plans are drawn and approved is far more expensive than moving it on a site plan before the assessment is done.
Common Mistakes
Assuming the main house’s BAL rating applies to the granny flat. The secondary dwelling is assessed independently at its own footprint. It may rate higher or lower than the main house depending on its position relative to vegetation and slope. Always commission a separate assessment for the granny flat location.
Using an old BAL assessment. Vegetation regrows. A BAL assessment done two years ago for the main house does not reflect current conditions, and most states do not accept assessments older than 12 months for a new building permit. In WA, assessments expire after 12 months by regulation.
Budgeting for construction before knowing the BAL rating. A 60-square-metre granny flat that costs $150,000 at BAL-LOW can cost $175,000 to $200,000 at BAL-29 or $200,000 or more at BAL-40. Getting a quote without knowing the rating means getting a quote that will change – sometimes significantly.
Choosing the site for convenience, not compliance. The obvious spot for a granny flat – at the back of the block, close to the garden, tucked under the trees – is often the worst spot for bushfire compliance. Trees and slope drive the rating. Siting the dwelling further from vegetation, even if it means a less picturesque location, can save tens of thousands of dollars and avoid a rating that restricts or prohibits the build entirely.
Confusing fire separation from the main house with bushfire construction. The NCC requires fire-rated construction between buildings that are close together – typically within 1.8 metres of a boundary or 900 mm of another building. This is fire separation under the Building Code, not bushfire compliance under AS 3959. They are different requirements, assessed differently, and both may apply to a secondary dwelling simultaneously.
Not checking whether the bushfire-prone area designation has changed. State bushfire mapping is updated periodically. A block that was outside the designated area when the main house was built may now be inside it, or vice versa. Check the current mapping – not an old planning certificate or the neighbour’s experience – before making assumptions about whether a BAL assessment is required.
Worked Example
A property owner in a semi-rural area of NSW proposes a 60-square-metre, two-bedroom granny flat for an ageing parent. The block is 2,000 square metres, designated bushfire-prone, with tall eucalypt woodland along the rear boundary.
Step 1 – Check the designation. The owner confirms via the NSW Planning Portal’s bushfire-prone land map that the property is within a designated area. A BAL assessment is required.
Step 2 – Commission the BAL assessment early. A Level 3 BPAD-accredited assessor inspects the site and tests two footprint locations. The first, 12 metres from the rear tree line, rates BAL-40 due to tall forest on an upslope. The second, 30 metres from the tree line and partially shielded by the main house, rates BAL-19.
Step 3 – Choose the lower-rated site. The owner selects the BAL-19 location. This keeps the project within the CDC pathway (BAL-19 is below the BAL-40 threshold) and saves an estimated $19,000 to $35,000 in construction costs compared to the BAL-40 site.
Step 4 – Approval. Because the rating is BAL-19, the owner applies for a Complying Development Certificate through a private certifier. The CDC is issued within 15 business days. No RFS referral is required.
Step 5 – Construction. The builder prices the job at $165,000 including BAL-19 compliance measures (non-combustible cladding, 5 mm toughened glazing, sealed building envelope, ember screens). The base cost without bushfire compliance would have been approximately $150,000 – a $15,000 or 10 per cent premium.
Step 6 – Final inspection. The building certifier verifies that all BAL-19 construction measures are installed as specified before issuing the occupation certificate.
Total BAL-related costs. BAL assessment $800, construction premium $15,000. Total additional cost for bushfire compliance: approximately $15,800 – meaningful, but manageable, and $20,000 to $35,000 less than the alternative site would have required.
Frequently Asked Questions
Do all granny flats in bushfire-prone areas need a BAL assessment? Yes. Under the NCC’s Part G5, every new Class 1a building (including secondary dwellings) on designated bushfire-prone land must have a BAL assessment before a building permit can issue. There is no exemption based on the size of the dwelling.
Can I build a granny flat in a flame zone (BAL-FZ)? It depends on the state. In NSW, the Rural Fire Service has a general policy position of not supporting new secondary dwellings in flame zones, making approval extremely difficult. In Victoria, a flame-zone build requires CFA referral and specific engineering solutions but is not outright prohibited. Other states assess on a case-by-case basis, but BAL-FZ construction costs are substantially higher and some suppliers will not quote for flame-zone work on small dwellings.
Will the BAL rating be the same as my main house? Not necessarily. The BAL rating is calculated at the building footprint, considering the specific distance and direction from that footprint to surrounding classified vegetation. Because a granny flat is sited at a different location on the block, it can rate higher or lower than the main house.
How much does a BAL assessment cost? Typically $400 to $2,000, depending on site complexity, the number of vegetation types present, and whether a site visit is required (it almost always is). The assessment must be carried out by an accredited assessor – Level 3 BPAD in NSW, FPAA-accredited in WA, or equivalent in other states.
Can I reduce my BAL rating by clearing vegetation? Potentially, but with significant limitations. Clearing on your own land may reduce the rating by increasing the separation distance, but you cannot clear vegetation on neighbouring land, in conservation areas, or where local tree preservation orders apply. Additionally, the BAL assessment considers vegetation within 100 metres of the building – much of which may be on neighbouring properties, road reserves, or public land that you have no control over. Discuss vegetation management options with your BAL assessor before counting on a reduced rating.
Does a granny flat in a bushfire zone affect insurance? Most insurers require disclosure of a BAL rating when insuring a dwelling. A higher BAL rating may increase premiums, and some insurers impose restrictions or exclusions for properties in higher BAL categories. If the granny flat will house a tenant, landlord insurance with bushfire coverage should be arranged. The BAL rating, the construction standard achieved, and the maintenance of any required Asset Protection Zone all factor into the insurer’s assessment.
Glossary
AS 3959. Australian Standard 3959 – Construction of buildings in bushfire-prone areas. The national standard that specifies construction requirements for each BAL level.
Asset Protection Zone (APZ). A managed area around a building where vegetation is reduced in density and height to lower bushfire risk. Width varies by state and BAL rating.
BAL (Bushfire Attack Level). A rating from BAL-LOW to BAL-FZ that classifies a building site’s exposure to ember attack, radiant heat and flame contact during a bushfire. Determined by an AS 3959 assessment.
BMO (Bushfire Management Overlay). A planning overlay used in Victoria that triggers planning permit requirements and CFA referral for development in high-risk bushfire areas.
BPA (Bushfire-Prone Area). Land designated by the relevant state authority as being subject to bushfire risk. The designation triggers BAL assessment and construction requirements under the NCC.
BPAD (Bushfire Planning and Design). An accreditation framework for bushfire consultants, primarily in NSW. Level 3 BPAD accreditation is required for BAL assessments used in development applications.
CDC (Complying Development Certificate). A fast-track approval pathway in NSW for development that meets predetermined standards. Available for secondary dwellings at BAL-29 or below.
Class 1a building. Under the NCC, a detached or attached dwelling. Granny flats and secondary dwellings are classified as Class 1a.
Classified vegetation. Vegetation that has been identified and categorised under AS 3959 into one of seven groups (forest, woodland, shrubland, scrub, mallee/mulga, rainforest, grassland) for the purpose of BAL assessment.
Defendable space. The area surrounding a building that provides a buffer from bushfire and gives occupants and firefighters a safer area from which to defend the property. Similar to an APZ.
FDI (Fire Danger Index). A numerical measure of fire weather severity for a given region, used in BAL calculations. The FDI is a fixed value determined by the geographic location, not measured on site.
NCC Part G5. The section of the National Construction Code that sets requirements for construction in bushfire-prone areas, referencing AS 3959 as the primary compliance standard.
Radiant heat. Electromagnetic radiation emitted by a bushfire, measured in kilowatts per square metre (kW/m2). The BAL rating thresholds correspond to specific radiant heat flux levels at the building.
Shielding. Under AS 3959 Clause 3.5, the reduction in radiant heat exposure provided by a non-combustible building or structure located between the fire source and the building being assessed.
Sources
- Standards Australia. AS 3959:2018 – Construction of buildings in bushfire-prone areas.
- Australian Building Codes Board. National Construction Code 2022, Volume Two, Part G5 – Construction in bushfire-prone areas.
- NSW Rural Fire Service. Planning for Bush Fire Protection 2019.
- NSW Department of Planning and Environment. State Environmental Planning Policy (Affordable Rental Housing).
- Country Fire Authority (Victoria). Planning and building in the Bushfire Management Overlay.
- Victorian Building Authority. Building in designated bushfire-prone areas.
- Queensland Fire and Emergency Services. Building in bushfire hazard areas.
- Country Fire Service (South Australia). Planning and building in bushfire areas.
- Department of Fire and Emergency Services (Western Australia). Bushfire-prone areas – building requirements.
- Tasmania Fire Service. Building in bushfire-prone areas.
- Fire Protection Association Australia. BAL assessor accreditation framework.
- CSIRO. Bushfire best practice guide – siting tutorial.
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