Flood, bushfire and heritage are the three constraints that most often turn a duplex site from feasible to unbuildable in NSW, and all three can be checked before you exchange. Flood is fatal when the land sits in a floodway or a high hazard category, bushfire is fatal when the asset protection zone leaves no room for a second dwelling, and heritage is fatal when the existing house is a listed item or a contributory building that the council will not let you demolish. Each has a survivable version too, and the job of due diligence is to tell the two apart.
Key takeaways
- Flood affectation is assessed under clause 5.21 of the LEP and the council flood study; a “flood affected” flag is not the same as an unbuildable site.
- Bush fire prone land triggers Planning for Bush Fire Protection 2019, and the asset protection zone is measured from the dwelling, not the boundary.
- Heritage items and conservation areas are controlled by clause 5.10 of the LEP, and demolition of a contributory building is rarely supported.
- All three constraints can remove complying development as an option, pushing the project to a DA.
- The Low and Mid-Rise Housing reforms exclude heritage items and certain hazard-affected land, so the new standards do not rescue a constrained site.
How the three constraints compare
| Constraint | Where it is mapped | Usually survivable when | Usually fatal when |
|---|---|---|---|
| Flood | LEP flood planning map, council flood study, 10.7 certificate | Flood fringe with floor levels achievable above the flood planning level | Floodway, flood storage or high hazard category |
| Bushfire | Council bush fire prone land map certified by the RFS | Vegetation buffer allows a BAL-29 or lower outcome with a compliant APZ | BAL-FZ or an APZ that consumes the second dwelling’s footprint |
| Heritage | LEP heritage map and Schedule 5, State Heritage Register | Site is in a conservation area but the house is not contributory | The house is a listed item or a contributory building that must be retained |
Flood: the difference between affected and unbuildable
Every NSW LEP now carries clause 5.21 (Flood planning), which applies to land at or below the flood planning level, usually the 1 percent annual exceedance probability flood plus a freeboard of 0.5 metres. Some councils also adopt clause 5.22 (Special flood considerations) for land between the flood planning level and the probable maximum flood, which matters most in the Hawkesbury-Nepean valley under Hawkesbury LEP 2012 and Penrith LEP 2010.
Being inside the flood planning area is common across Penrith, Hawkesbury, parts of Blacktown along South and Eastern Creeks, and the Georges River councils. It is survivable when the council’s flood study puts the site in the flood fringe, the required floor level can be reached without an unreasonable building height, and there is a safe evacuation route. It becomes fatal when the site is in a floodway or flood storage area, where intensifying the use is normally refused, or in a high hazard category where the depth and velocity make the land unsafe for people and structures.
Flood also affects the pathway. The Codes SEPP treats a “flood control lot” specially, and complying development is only available where the lot meets the flood-related conditions in the relevant code, which usually means a DA for anything in a floodway or above a modest depth. Our flood engineering colleagues at Contrive Consultants prepare the flood impact assessment where a DA needs one. The NSW Government’s overview is on the Department of Planning’s hazards page.
Bushfire: the asset protection zone decides the yield
If the parcel is on the council’s bush fire prone land map, section 4.14 of the Environmental Planning and Assessment Act 1979 requires the consent authority to be satisfied the development conforms to Planning for Bush Fire Protection 2019. The NSW Rural Fire Service explains how the mapping works on its bush fire prone land page.
The assessment produces two numbers that shape a duplex. The asset protection zone (APZ) is the managed buffer between the vegetation and the dwelling, and it must sit within the lot or on land the owner controls. The bushfire attack level (BAL) sets the construction standard under AS 3959:2018, from BAL-12.5 through to BAL-FZ (flame zone). A rear dwelling on a fringe lot in the Hills, Hawkesbury, Wollondilly or the Blue Mountains often lands closer to the vegetation than the existing house, which pushes it into a higher BAL and demands a larger APZ. When the APZ swallows the footprint, the second dwelling cannot be placed at all.
Two further points. BAL-40 and BAL-FZ outcomes generally close off complying development, so the project becomes a DA. And where the proposal involves subdividing bush fire prone land for residential purposes, it is integrated development under section 100B of the Rural Fires Act 1997 and needs a Bush Fire Safety Authority from the RFS, which adds time and can add conditions on access and water supply.
Heritage: when the existing house is the problem
Clause 5.10 (Heritage conservation) of every Standard Instrument LEP controls works to heritage items and to land within a heritage conservation area (HCA). The two are different. An item is individually listed in Schedule 5 of the LEP and its demolition is close to impossible to justify. An HCA protects the character of a precinct, and the test for demolition of a building inside it is whether the building contributes to that character.
A duplex on a listed item is very rarely approved unless the item is retained and the second dwelling is a sympathetic addition at the rear. A duplex in an HCA can work where the existing house is non-contributory, or where the design keeps the front of a contributory house and places the second dwelling behind it, but a side by side pair of new dwellings replacing a period cottage is usually refused. Parramatta, Cumberland, Canterbury-Bankstown, Newcastle and Wollongong all have large HCAs, and each requires a heritage impact statement with the DA.
Heritage removes complying development altogether, since the Codes SEPP excludes heritage items and, for most codes, land in an HCA. It also sits outside the Low and Mid-Rise Housing reforms, which exclude heritage items from the new standards, so the reforms that commenced on 28 February 2025 do not change the answer on a listed site.
How we test a site against all three
- Read the 10.7 certificate. It answers yes or no on flood controls, bush fire prone land and heritage in one document, and its complying development section tells you which codes are already excluded.
- Pull the maps. The LEP flood, heritage and lot size maps on the Planning Portal, and the council flood study for the hazard category.
- Place the second dwelling on paper. Draw the APZ and the flood planning level onto the survey and see what footprint is left.
- Check the neighbours. A duplex approved two doors down on the same flood category or BAL tells you the council’s current position.
- Decide the pathway. If any constraint removes the CDC option, the DA program and the specialist reports go into the feasibility from the start.
On Hawkesbury-Nepean valley sites the flood category usually settles the question in the first hour. Our Hawkesbury town planning page covers the local flood clauses in more detail.
Frequently asked questions
Can you build a duplex on flood affected land in NSW?
Often yes, if the land is in the flood fringe and the habitable floors can be set above the flood planning level with safe access. It is usually refused in a floodway, flood storage area or high hazard category. The council flood study and clause 5.21 of the LEP decide which applies.
Can you build a duplex on bushfire prone land?
Yes, provided both dwellings meet Planning for Bush Fire Protection 2019 with a compliant asset protection zone and are built to the assessed BAL under AS 3959:2018. A BAL-40 or flame zone outcome for the rear dwelling usually means a DA and a much higher build cost, and sometimes no second dwelling at all.
Can you knock down a house in a heritage conservation area for a duplex?
Only where the council accepts the house is non-contributory to the conservation area. If the house is contributory, expect to retain it and add the second dwelling behind. If it is a listed item, demolition is almost never supported.
Do the 2025 low and mid-rise housing changes apply to flood or heritage land?
No. The reforms exclude heritage items and land affected by hazards such as flooding and bushfire, so the new non-refusal standards do not apply. Those sites are assessed under the LEP and DCP through a normal DA.
URBA’s planners test flood, bushfire and heritage before you commit to a duplex site, and prepare the SEE and coordinate the specialist reports when the site can carry two dwellings. See our development due diligence service and our duplex planning page, or request a fee proposal with the address.