A flood overlay means the planning maps show your land could be affected in a defined flood, usually the one with a 1% chance of happening in any year. It does not stop you living there or building there. It changes how you build: how high the floor has to be, what materials go below the flood level, and sometimes which approval route you can use.
It also works the other way. A property with no flood overlay is not necessarily safe from flooding, and that is the part most people get wrong.
What the mapping is based on
Most flood overlays are drawn from a council or state flood study. The study models how water moves through a catchment and maps the land covered in a chosen event.
- The 1% AEP flood. AEP stands for annual exceedance probability. A 1% AEP flood has a one in 100 chance of happening in any given year, which is why it is often called the 100-year flood. It is the usual basis for planning controls.
- Freeboard. A safety margin added on top of the modelled flood level. In NSW, the Flood Planning Area in a council’s Local Environmental Plan is the 1% AEP flood plus freeboard, under clause 5.21.
- The probable maximum flood. The largest flood that could reasonably occur. Some councils map it as well, and in NSW clause 5.22 applies extra considerations to sensitive and hazardous development on that land.
- Overland flow. Stormwater that runs across the ground in heavy rain, away from any creek or river. In cities this is often what floods houses, and it is mapped separately where it is mapped at all.
What it is called in each state
| NSW | The Flood Planning Area in your council’s Local Environmental Plan, drawn from council flood studies. Not every council has published mapping. |
| Victoria | The Land Subject to Inundation Overlay (LSIO), the Floodway Overlay (FO) and, for overland flow in urban areas, the Special Building Overlay (SBO). |
| Queensland | A flood hazard overlay in your council’s planning scheme. The state also publishes the Queensland Floodplain Assessment Overlay, a broad-scale map its publisher says is not intended to predict flooding for individual properties. |
| South Australia | The flooding overlays in the Planning and Design Code. |
| Western Australia | No statewide planning layer. Flood mapping is held by councils and by the Department of Water and Environmental Regulation. |
| Tasmania | The Flood-prone Hazard Areas Code in the planning scheme, which covers river flooding. Coastal inundation has its own code. |
| ACT | A modelled 1% AEP flood extent covering eight catchments, which is a small share of the Territory. |
| Northern Territory | Storm surge areas on the coast are mapped. River flood studies are published as maps rather than a planning layer. |
What changes when you build
Flood controls are set by your council, so the detail varies, but these are the usual ones:
- A minimum floor level. Habitable rooms usually have to sit above the flood level plus freeboard. On a low block that can mean a raised floor, which affects the design and the cost.
- Flood-compatible materials for anything built below that level.
- Limits on filling and fencing that would block or divert floodwater onto neighbours.
- The approval route. In NSW, flood-affected land can rule out the fast-track complying development route for granny flats, sheds and decks, so the project needs a development application instead.
Your council can usually give you the flood level for your property. Many NSW councils provide this as a flood information certificate or flood advice letter, and in NSW the planning certificate for a property (a section 10.7 certificate) states whether flood-related development controls apply.
No overlay does not mean no flood risk
Flood mapping only exists where someone has done a flood study and published it. Large parts of Australia have no published mapping at all. In NSW, many councils have no flood mapping in a form that can be checked online, and in those areas a property shows no flood overlay because nothing is mapped, not because the land is safe.
Mapping can also be old, and it often covers rivers and creeks but not overland flow. If a property has flooded before, or sits low in the street, ask the council what studies exist and talk to the neighbours.
ZoneScout reports these gaps instead of hiding them. Where no flood source covers your area, a search says the flood check could not be done, so a missing map never reads as a clear result.
Flood overlays and insurance
Insurers use their own flood data, which is not the same as the planning maps. A property can carry a high flood premium with no planning overlay, or the other way round. If you are buying, get an insurance quote that includes flood cover before you exchange.
Frequently asked questions
What does 1% AEP mean?
It means a flood with a 1% chance of happening in any given year, often called the 100-year flood. It is not a once-a-century event: over 30 years, the chance of at least one such flood is about one in four.
Can I build on land with a flood overlay?
Usually, yes. A flood overlay sets conditions such as a minimum floor level and flood-compatible materials, and it can rule out the fast-track approval route. It rarely stops building outright, but it affects the design and the cost.
Does a flood overlay affect insurance?
Not directly. Insurers use their own flood data, so a premium can reflect flood risk whether or not a planning overlay applies. Get a quote with flood cover before you buy.
Why does my property show no flood overlay when the street has flooded?
Because the mapping may not exist, may be out of date, or may only cover rivers and creeks and not overland flow. Ask your council what flood studies cover your area.
Check your own property
See the zone, overlays and lot size for any Australian address, free.