Flood Overlays and Flood-Prone Land Explained

Flood-prone land is a significant issue for new home builds in many parts of Australia. If your block is in a flood overlay, the council will impose conditions on how the building must be constructed — typically requiring the finished floor level to be set above the designated flood level with a freeboard. These conditions can add thousands or tens of thousands of dollars to the build cost.

Understanding flood overlays, flood levels, and the construction requirements is essential if you are buying land in an area that may be flood-prone. The cost of building on flood-prone land can be significantly higher than on a standard block, and the insurance implications can affect your ongoing costs for as long as you own the home.

What Is a Flood Overlay?

A flood overlay is a planning control that applies to land identified as being at risk of flooding. The overlay is typically based on the 1-in-100-year flood event (the flood that has a 1% chance of being equalled or exceeded in any given year, also called the 1% AEP). Some councils also use a more conservative flood event (e.g. the 1-in-200-year or 0.5% AEP flood) for critical infrastructure.

The flood overlay is shown on the council's planning scheme maps and is typically identified by a code such as 'Flood Overlay', 'Land Subject to Inundation Overlay (LSIO)', or 'Special Building Overlay (SBO)' depending on the state and council.

Flood Levels and Freeboard

When a block is in a flood overlay, the council (or the relevant flood authority) provides a flood level for the site. This is the expected water level during the designated flood event (typically the 1% AEP flood). The building must be constructed with the finished floor level (FFL) above this flood level, plus a freeboard.

The freeboard is an additional height above the flood level to account for wave action, debris, and uncertainty in the flood modelling. The freeboard is typically 300–500mm, but varies by council.

For example, if the flood level is 10.0m AHD (Australian Height Datum) and the freeboard is 500mm, the required finished floor level is 10.5m AHD. If the natural ground level is 9.5m AHD, the building needs to be raised by 1.0m.

How Flood-Prone Land Affects Build Cost

Building on flood-prone land can add cost in several ways:

Insurance Implications

Building on flood-prone land can significantly affect your insurance premiums. Most home insurance policies in Australia include flood cover, but the premium is based on the flood risk of the property. A property in a flood overlay with a high flood level above the natural ground may have premiums that are several times higher than a comparable property outside the flood overlay.

In some cases, insurers may refuse to provide flood cover for properties in high-risk flood areas, or may impose very high excesses for flood claims. Before buying land in a flood overlay, obtain a flood insurance quote to understand the ongoing cost.

Some states have a flood insurance comparison tool or a flood insurance fact sheet that can help you understand the likely premium impact. Check with your state's emergency services or insurance council.

How Builders Handle Flood Costs in Contracts

Builders handle flood-related costs in one of three ways:

• **Included in the site costs** — The builder has checked the flood level and included the cost of raising the building, importing fill, or using a suspended slab in the fixed site costs. This is the best outcome but requires the builder to have confirmed the flood level.
• **Provisional sum** — The builder includes an estimate for flood-related costs but notes that the actual cost may vary. This is common when the flood level has not been confirmed at the time of quoting.
• **Exclusion** — The builder explicitly excludes all flood-related costs from the contract. You are responsible for arranging and paying for the flood-related construction, either through the builder as a variation or through a separate contractor.

What to Check Before Buying Land

Common Mistakes and Hidden Costs

• Not checking whether the block is in a flood overlay before buying.
• Assuming the builder's site costs include flood-related construction — many builders exclude this or use a provisional sum.
• Not obtaining the flood level before signing the building contract — the cost of raising the building can be significant.
• Forgetting that flood-prone land can have higher insurance premiums — obtain a quote before buying.
• Not checking whether fill can be imported to raise the ground level — some councils restrict fill in flood overlays to avoid displacing floodwater.
• Assuming a '1-in-100-year flood' means the flood happens once every 100 years — it means a 1% chance each year, so it could happen multiple times in a short period.

Frequently Asked Questions