Is my block flood affected?

Written by Shea Cullen, architect, for Dudils.

Flood mapping in New South Wales is held by several bodies, and their maps do not always agree. Type an address and the Site Checker reads each of them for your lot and shows them side by side: your council's own flood maps where we can read them, the flood planning map in the NSW Planning Portal, and the NSW SES flood extents. It never merges them into one verdict, and it is not a flood certificate.

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Is my property flood affected?

The Site Checker shows whether your lot sits inside any flood extent we can read, and names the map, the study and the year for each finding. A lot inside a council flood planning area, or inside the 1% AEP extent from any source, is flood affected for planning purposes and will carry flood controls on a new house or a large addition.

A lot outside every map we read is reported as not inside any mapped flood extent, which is a narrower statement than it sounds. Overland flow, a drainage study council has not published as a map, or a flood study still in progress can all still apply to it.

Coverage is uneven. We read the council's own layers for the councils listed at the bottom of this page, the Planning Portal's flood planning map wherever a council has put one in its Local Environmental Plan, and the SES extents across the state wherever a flood study exists. The report tells you which of those answered for your lot.

What does flood control lot mean?

A flood control lot is a lot where council applies flood related development controls to new homes, and the section 10.7 planning certificate says whether yours is one. The term comes from the Codes SEPP, the state policy for complying development.

Complying development can still be carried out on a flood control lot, but not on any part of it that is a floodway, a flood storage area, a flow path, or a high hazard or high risk area, unless council or a hydraulic engineer certifies that part is not. The work also comes with conditions: habitable floors at or above the flood planning level, flood compatible materials below it, a structure that can withstand the flood, no increase in flooding elsewhere, and access to a refuge. Certifiers will usually ask for council's flood information for the lot before they issue a certificate.

What is a 1% AEP or 1 in 100 year flood?

It is a flood with a 1% chance of being reached or exceeded in any single year. AEP stands for annual exceedance probability. The older name, the 1 in 100 year flood, leads people to think a century passes between them, but over 70 years the chance of at least one is about 50%, and two can arrive in consecutive years.

The 1% AEP flood is the usual basis for planning controls on homes in NSW. The probable maximum flood (PMF) is the largest flood that could reasonably occur, and it matters for evacuation and critical facilities more than for most house floor levels. The report shows a PMF finding separately when the SES extent covers your lot.

What floor level will I need to build to?

Usually the flood planning level: the 1% AEP flood level plus a margin called freeboard, typically 0.5 metres, set by council. The NSW Flood Risk Management Manual (2023) is the state guide councils follow when they set it, and your council's DCP states the freeboard and which rooms must sit above it.

The Site Checker does not give a flood planning level, because no free source publishes one in metres AHD (the Australian Height Datum, roughly mean sea level) for each lot. It comes from council's flood model. Ask council for a flood certificate or flood information report, which usually states the flood levels at the lot, and have a surveyor measure your ground levels to compare. A floor that has to rise a metre or more changes the design, the steps to the front door and the cost, so it is worth knowing before you buy.

Does a flood map affect insurance?

It can, but insurers use their own flood data rather than the council map. Most home insurers price flood from the Insurance Council of Australia's National Flood Information Database, which draws on council flood studies, together with their own models. A reading on the Site Checker gives you a reason to ask, not a premium. Get a quote for the actual address before you exchange contracts, and ask whether flood is covered and at what excess.

My council map says one thing and the state map another. Which is right?

For a development application, the council's adopted flood planning map and its flood information are what the house is assessed against, and the planning certificate is the legal statement. The maps differ for ordinary reasons: they come from different studies, run in different years, for different purposes. The SES extents show where a design flood reaches, for emergency planning. A council flood planning area adds freeboard, so it usually reaches further up the slope. A former LEP map may have been replaced by a newer study.

That is why the Site Checker shows each source with its own name and date and never merges them. Where they disagree, ask council which study it now relies on.

Where do I get the official answer?

From council. The section 10.7 planning certificate states whether the lot is subject to flood related development controls, and many councils issue a flood certificate or flood information letter with the flood levels at the lot. Neither the Site Checker nor any other free map replaces them.

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What else do people ask?

Does a clear reading mean my block will never flood?

No. It means none of the maps we read shows the lot inside a mapped flood extent. Overland flow, blocked drains and floods larger than the design flood are not on these maps.

What is overland flow flooding?

Water running over the ground after heavy rain, before it reaches a creek or a drain. Many older flood studies do not map it, and some councils now map it separately from river and creek flooding.

Can I build on flood prone land?

Often, with conditions on floor levels, materials and access. Land in a floodway or a high hazard area is much harder to build on, and complying development is not available there.

Which council flood maps does the Site Checker read?

For these councils the report reads the council's own flood layers as well as the state maps. Each page explains what that council publishes and how to get its flood information.

Where does this information come from?

The Site Checker reads public maps. It is not a section 10.7 planning certificate, a flood certificate, a bushfire attack level assessment or a survey, and a map can be out of date for your lot. Confirm anything you rely on with council or the relevant authority before you buy, design or build. Nothing on this page replaces advice from your certifier, engineer, lawyer or your own designer.