Stormwater Requirements for New Homes

When you build a new home, you create a large area of hard surfaces — the roof, the driveway, the patio, the paths — that rainwater cannot soak into. This water has to be managed. In Australia, every council has stormwater requirements that dictate how rainwater runoff from a new home must be collected, controlled and disposed of.

The requirements vary by council and by site, but they generally fall into three categories: connection to the council stormwater system, on-site dispersion, and on-site detention. Understanding which applies to your site, and whether the cost is included in your builder's fixed price, can prevent a surprise bill.

The Three Types of Stormwater Management

On-Site Detention (OSD) Explained

On-site detention is the most expensive stormwater requirement. An OSD system consists of a large underground tank (or above-ground tank) that collects stormwater from the roof and hard surfaces, holds it during a rain event, and slowly releases it into the council stormwater system or a dispersion area at a controlled rate.

The purpose of OSD is to prevent the council stormwater system from being overwhelmed during heavy rain. By holding the water on your site and releasing it slowly, you reduce the peak flow into the council system, which helps prevent downstream flooding.

OSD is commonly required in:

• Urban infill developments (where the new home is larger than the old home it replaces, increasing runoff)
• Areas with an overloaded council stormwater system
• Flood-prone or low-lying areas
• New estates where the developer has not provided sufficient stormwater infrastructure
• Sites where the council has a policy of requiring OSD for all new builds

An OSD system typically includes:

• A detention tank (2,000–10,000+ litres, depending on the roof area and council requirements)
• A controlled flow restriction device (to limit the outflow rate)
• An overflow path (to direct excess water safely away from the house)
• Connection piping from the roof and hard surfaces to the tank
• Discharge piping from the tank to the council system or dispersion area

How to Check Your Council's Stormwater Requirements

Before you sign a building contract, you should check what stormwater requirements apply to your site. The best way to do this is:

1. Check the council's development requirements on their website — most councils publish their stormwater requirements online.
2. Look for a stormwater management plan or a drainage plan for the estate (if you are building in a new estate, the developer may have already addressed stormwater).
3. Ask the builder whether they have checked the council's stormwater requirements for your site and whether any special requirements (like OSD) apply.
4. Check the contract documents for a stormwater plan or a stormwater condition — if the council has issued a condition requiring OSD, it should be listed.
5. If you are unsure, contact the council's planning or engineering department and ask what stormwater requirements apply to a new build on your block.

How Builders Handle Stormwater in Contracts

Builders handle stormwater costs in one of three ways:

• **Included in the site costs** — The builder has checked the council requirements and included the stormwater system (connection, dispersion, or OSD) in the fixed site costs. This is the best outcome.
• **Provisional sum** — The builder includes an estimate for stormwater but notes that the actual cost may vary, particularly if OSD is required and the final design has not been confirmed.
• **Exclusion** — The builder explicitly excludes the stormwater system from the contract. You are responsible for arranging and paying for the stormwater system, either through the builder as a variation or through a separate contractor.

Stormwater and Sloping Blocks

On a sloping block, stormwater management is more complex because the water naturally flows downhill. The council may require you to manage stormwater to prevent it from flowing onto neighbouring properties or causing erosion.

On a sloping site, additional stormwater costs may include:

• Surface drains (swales) to direct water away from the house
• Subsurface drains (agricultural pipes) to manage groundwater
• Retaining wall drainage (agricultural pipes behind retaining walls)
• Erosion control measures (sediment fences, rock check dams) during construction
• Discharge point management (ensuring water is released at a controlled point, not onto a neighbour's land)

What to Check Before Signing

Common Mistakes and Hidden Costs

• Assuming stormwater is included in the builder's base price when it is actually a provisional sum or exclusion.
• Not checking whether the council requires an OSD system before signing — this can add $10,000+ to the build cost.
• Forgetting that sloping blocks need additional drainage (surface and subsurface drains).
• Not checking whether the connection to the council stormwater main is included or is a separate cost.
• Assuming the builder has checked the council requirements — many builders use a standard stormwater allowance that does not account for site-specific requirements.
• Forgetting that retaining wall drainage (agricultural pipes behind retaining walls) may be a separate cost from the retaining wall itself.

Frequently Asked Questions