Stormwater is the rain that falls on your land and must leave it safely. The legal principle that drives nearly every Australian council's planning regime is simple: post-development runoff must not exceed pre-development runoff, and downstream neighbours must not be worse off. As soon as you build a roof, a driveway and any paving, your lot is producing more, faster runoff than it did as bare ground. Managing that difference is a discipline of its own, and on sloping sites it can be one of the most consequential design questions on the block.

The main mechanism is on-site detention (OSD) — a tank or basin that temporarily stores peak stormwater and releases it slowly through a small orifice plate. OSD doesn't reduce total water leaving the lot; it just stretches the discharge over time so the downstream pipe and waterway can cope. Councils set their Permissible Site Discharge (PSD) — the maximum litres per second your lot can release, typically 20–80 L/s/ha — and a corresponding Site Storage Requirement (SSR) — the minimum volume your tank must hold, often 300–500 m³ per hectare. The OSD is sized to those parameters.

OSD is distinct from a rainwater tank, which is retention (water stored for use) rather than detention. Most modern Australian planning regimes encourage or mandate rainwater tanks. BASIX in NSW typically requires them. Many councils elsewhere accept a single combined tank that meets both functions.

Charged stormwater systems become necessary when the lot sits below the level of the council's stormwater pit — i.e., gravity drainage to the street kerb is impossible. A charged system uses sealed pressurised pipework that allows water to exit at the level required. Charged systems work but add cost and ongoing maintenance — and they only work if installed correctly.

Water Sensitive Urban Design (WSUD) is the integrated approach: permeable paving, raingardens, bioretention basins, swales, infiltration trenches. WSUD reduces peak flows, improves water quality before runoff reaches the public drain, and gives some of that water back to the soil and to landscaping. Many councils now require WSUD elements on new developments.

Where it intersects with other rules: BASIX (NSW) and the National Construction Code 2022 Part H7 (which applies nationally to Class 1 and 10 houses) integrate water efficiency with energy efficiency. The NatHERS thermal scheme handles the energy side. A BASIX certificate is typically $50–$200 to obtain; compliance costs (tanks, glazing, insulation) can add $3,000–$15,000 to a build.

Indicative 2025–26 costs: a residential rainwater tank, $2,000–$6,000 installed (5,000 L typical); an OSD tank, $6,000–$25,000 installed for typical residential sizes; a charged stormwater system, $3,000–$12,000 above conventional drainage; permeable paving premium, $80–$200/m² above conventional paving; raingarden or bioretention basin, $5,000–$25,000 depending on size; hydraulic engineering design, $2,500–$6,000.

Questions worth asking the seller (and council):

  • Where is the legal point of discharge for this lot — front kerb, rear easement, council pit?
  • Does the lot sit below kerb level, requiring a charged or pump-out system?
  • Will OSD be triggered, and at what storage volume and PSD?
  • Is a rainwater tank required (BASIX or council policy), and can it be combined with OSD?
  • Are there easements to drain water to or from neighbouring lots (cross-reference Post 5)?
  • What permeable-surface percentage applies to this lot?
  • Are WSUD treatments required, and what are their ongoing maintenance obligations?
  • Will the council require a registered positive covenant for OSD maintenance?
  • Are there overland flow paths across the lot that need to be accommodated?

Who can help. A hydraulic or stormwater engineer (CPEng/NER, RPEQ in Queensland) — $2,500–$8,000 for residential design. A civil engineer for subdivision-scale stormwater — $5,000–$25,000+. A landscape architect for WSUD/bioretention design — $3,000–$12,000. A BASIX or NatHERS-accredited assessor in NSW — $400–$1,200.

Two practical points. First, charged systems generate ongoing maintenance liabilities. The lot owner is responsible for keeping the trash screen, orifice plate and pipework clear — and many councils impose a positive covenant making annual inspection mandatory. Factor that into long-term cost. Second, stormwater is often the most council-specific control on the lot. Two adjoining suburbs in different LGAs can have very different stormwater rules. Always pull the council's stormwater DCP for the specific lot before assuming standard practice applies.

This article is general information only — a starting point for your own questions, not hydraulic engineering or stormwater design advice. Stormwater requirements vary significantly by council and catchment, and recent national construction code provisions (NCC 2022 H7) and state-based schemes (e.g. BASIX) are subject to ongoing revision. Always engage a Chartered Professional Engineer with hydraulic experience, and request from the seller any stormwater plans, OSD calculations or BASIX certificates already prepared for the lot. Independent advice should be obtained before making any property decision.