OSD, Infiltration or Rainwater Retention: Which Suits Your Site?

Every development site in Greater Sydney has to answer the same question: when rain falls on the new roof and driveway, where does the extra water go, and how fast? There are three broad ways to answer it — detain it and release it slowly (on-site detention), soak it into the ground (infiltration), or capture and reuse it (rainwater retention). Builders often assume the choice is the engineer’s preference, or worse, that any of the three will do. In reality, the decision is usually made by two things nobody on the project controls: the soil under the site and the council’s DCP.

Pick the wrong strategy — or let the architect fix the site layout before the strategy is settled — and the result is familiar: a redesign at DA stage, a tank squeezed into the last unexcavated corner of the site, or an RFI asking for geotechnical evidence that was never commissioned. This article walks through how each strategy actually works, where each one is viable in Sydney conditions, and the decision factors a builder or developer should understand before the concept plans harden.

Three Strategies, Three Different Jobs

On-site detention: managing peak flow

On-site stormwater detention (OSD) temporarily stores runoff during a storm and releases it through a throttled outlet at a controlled rate. The design revolves around two numbers: the permissible site discharge (PSD), the maximum rate your site may release, and the site storage requirement (SSR), the volume you must hold to stay under it. In the Parramatta and Hills districts many councils still reference the UPRCT On-Site Stormwater Detention Handbook for these values; other councils set their own rates or require pre- versus post-development flow matching across a range of storm frequencies.

The key point: detention manages the rate of discharge, not the amount. Every litre that lands on the site still leaves the site — just more slowly, so the council’s downstream pipes and channels are not overwhelmed. OSD does nothing to reduce total runoff volume, which is why it is sometimes paired with the other two strategies.

Infiltration: managing volume

An infiltration system — trenches, soakage pits or modular cells wrapped in geotextile — disposes of runoff by letting it soak into the surrounding soil. Done well, it reduces both peak flow and total volume, mimicking how the site behaved before development. It can eliminate or shrink the connection to the street drainage system entirely, which is valuable on sites with no easy discharge point.

The catch is that infiltration is entirely hostage to the ground. It needs permeable soil, adequate separation from groundwater and structures, and a council willing to accept it. Where those conditions exist, it is often the most elegant answer. Where they do not, it fails — slowly, expensively and usually after handover.

Rainwater retention: capture and reuse

A rainwater tank captures roof runoff for reuse — toilets, laundry, irrigation. In NSW residential work this is usually driven by BASIX, the state’s sustainability scheme, which commonly commits a dwelling to a rainwater tank of a given size connected to specific end uses. Retention reduces demand on potable water and trims runoff volume from frequent small storms.

What a standard retention tank does not reliably do is detention. If the tank is full when the design storm arrives — and after a wet week in Sydney it often is — it provides zero storage for the event the OSD system is sized against. This is the single most common confusion on residential projects, and it is worth unpacking.

Retention vs Detention: The Distinction That Trips Everyone Up

Retention volume is water you intend to keep; detention volume is space you must keep empty. A 5,000-litre BASIX tank plumbed for reuse is retention. A council OSD requirement is detention. They are different volumes doing different jobs, and one cannot simply be counted as the other.

They can, however, share a vessel. Many councils accept a combined tank in which a dedicated air space above the reuse volume is reserved for detention: the lower portion stores water for reuse, while the upper portion sits empty behind a throttled orifice, filling only during storms and draining down afterwards. The detention zone must be genuinely dedicated — guaranteed empty at the start of the design storm — which is achieved by the outlet arrangement, not by optimism. Where a council accepts this, a single tank can satisfy both the BASIX commitment and the OSD condition, saving cost and site area. Where it does not, you need both volumes provided, sometimes in separate structures. Check before the hydraulic plans are drawn, not after.

Where OSD Is Mandated and Infiltration Is Allowed

Council policy is the first filter, and it varies across Sydney. Broadly:

OSD is the default requirement in most established Sydney catchments, where downstream drainage is at or near capacity. Councils in these areas mandate OSD for most new dwellings, dual occupancies and larger developments, with PSD/SSR values or modelling requirements set out in the DCP or engineering specification.
Infiltration is permitted or encouraged by some councils in areas with suitable soils, sometimes as a substitute for OSD and sometimes only as a supplement. Councils that allow it almost always require geotechnical evidence — permeability testing, groundwater observations — and impose minimum setbacks from buildings and boundaries.
Some councils prohibit infiltration outright in certain areas: reactive clay soils, salinity-affected land, steep slopes or landslip-prone terrain. In these locations proposing a soakage system is a fast way to collect a refusal.

The pattern to internalise is that the DCP decides more than personal preference does. An infiltration system that performed beautifully on a sandy site in one LGA is simply not an available option on a clay site two councils west, no matter how much the owner liked it last time.

Sydney Soils: The Ground Rules

Clay country

Much of Western Sydney — including the growth areas where most of the region’s new housing is being built — sits on clay-dominated soils derived from shale. Permeability is low, which means infiltration rates are poor and soakage systems either need enormous footprints or simply do not empty between storms. Reactive clays also shrink and swell with moisture, so deliberately concentrating water into the ground near footings is a structural risk as well as a drainage one. On most clay sites, OSD is not just the council’s requirement — it is the only strategy that physically works.

Salinity

Parts of Western Sydney are mapped as salinity-affected, with salt stored in the soil profile and groundwater. Deliberately recharging groundwater through infiltration can mobilise that salt, damaging building materials, infrastructure and vegetation. Councils in affected areas typically restrict or prohibit infiltration and may require salinity assessment as part of the DA. If your site is in a mapped salinity area, plan on detention.

Sandy soils

Sydney’s sandier soils tell a different story — permeability can be excellent, and infiltration becomes genuinely attractive where council policy and groundwater levels allow. Even then, testing matters: sand over a shallow clay lens or a high water table behaves nothing like the textbook, and a soakage pit that sits in groundwater for half the year is just an expensive hole.

Test, don’t assume

The soil maps give a starting point, not a design. Any serious infiltration proposal needs site-specific permeability testing to establish the actual soakage rate, plus groundwater observations. The cost of the testing is trivial against the cost of a failed system — or a redesign when council asks for the evidence you skipped.

Combined Systems and How the Decision Actually Gets Made

Real projects frequently combine strategies. A dual occupancy might carry a BASIX rainwater tank with a dedicated detention air space; a larger site might pair an OSD tank with infiltration cells that bleed off frequent small storms; a site with marginal soakage might use infiltration for volume with a detention system as the backstop for major events. The right combination falls out of a consistent set of questions:

Soil: What is the measured permeability, and is the site salinity- or slip-affected? This decides whether infiltration is even on the table.
Slope: Steep sites complicate infiltration (water re-emerging downslope) and shape where a detention tank and its outlet can sit.
Groundwater: A high water table rules out meaningful soakage and can affect underground tank construction.
Council policy: What does the DCP mandate, permit and prohibit? What PSD/SSR method applies? Will a combined retention-detention tank be accepted?
Discharge point: Is there a street gutter, drainage pit or easement at a workable level? A site that falls away from the street may need a charged system, a pump-out arrangement, an interallotment easement — or a strategy that minimises discharge altogether.

Notice what is not on the list: preference. The physics and the DCP make most of the decision; good engineering finds the cheapest compliant arrangement within those constraints.

Get the Engineer In Before the Layout Is Fixed

The most expensive stormwater problems are created at sketch design, long before an engineer is engaged. The architect places the building over the only viable tank location; the basement consumes the fall needed for a gravity outlet; the landscape plan claims the strip where the overland flow path must run. By DA lodgement the strategy is being reverse-engineered around a fixed layout, and the client pays for that in storage volume, pumps and redesign fees.

The fix costs almost nothing: a stormwater strategy review at concept stage. A capable engineer can look at the site survey, the soil landscape, the council’s DCP and the proposed yield, and tell you within days which strategy will be required, roughly what volume it implies and where it should sit. That single conversation routinely saves multiples of its fee — and for certifiers, a project that arrives with a coherent strategy is one that moves through construction certificate without drama.

Frequently Asked Questions

Can my BASIX rainwater tank count as my OSD?

Not by default. A BASIX tank provides retention — water stored for reuse — and may be full when the design storm arrives, so it offers no guaranteed detention. Many councils will accept a combined tank with a dedicated, always-empty air space for detention above the reuse volume, controlled by the outlet arrangement. Whether that is permitted, and how the volumes are calculated, depends on your council’s DCP, so confirm before the tank is specified.

Why won’t my council allow infiltration on my site?

Usually because of ground conditions. Much of Western Sydney sits on low-permeability, reactive clay where soakage systems do not drain and concentrated water can damage footings, and some areas are salinity-affected, where recharging groundwater mobilises salt. Councils restrict infiltration in these areas to protect buildings and infrastructure. Where soils are suitable, councils that allow infiltration will still require permeability testing and setbacks.

Do I always need OSD for a new dwelling or duplex in Sydney?

Not always, but often. Requirements are set by each council’s DCP and engineering standards, and they differ by development type, site area and catchment. Some councils exempt single dwellings below a threshold; others require OSD for virtually all new impervious area. The only reliable answer comes from checking the current DCP for your specific site — which a stormwater engineer can do in minutes at feasibility stage.

What testing do I need before proposing an infiltration system?

At minimum, site-specific permeability (soakage) testing by a geotechnical engineer to establish the design infiltration rate, plus groundwater observations to confirm adequate clearance below the system. In mapped salinity areas a salinity assessment may also be required. Councils generally will not accept an infiltration design based on soil maps or assumed rates alone.

Not sure which stormwater strategy your site and council will accept? Contrive Consultants advises builders and developers across Rouse Hill, Western Sydney and Greater Sydney on OSD, infiltration and combined retention-detention systems — ideally before the layout is locked in. Explore our stormwater engineering services or get in touch for a concept-stage strategy review — call +61 497 848 111 or email info@contriveconsultants.com.au.