What is On-Site Detention (OSD)? A Builder’s Guide for NSW
The short version
On-site detention (OSD) is a stormwater system that temporarily holds rainwater on your property during a storm and releases it slowly into the council drainage network. It doesn’t reduce the total amount of water leaving your site — it just slows it down, so the pipes and creeks downstream don’t get hit with everything at once.
If you’re building in Sydney, there’s a good chance your council will ask for an OSD system as a condition of development consent. This guide explains what you’re actually being asked to provide, without the jargon.
Why councils require OSD
When a block of land is developed, soft surfaces that used to soak up rain — lawn, garden, bare soil — get replaced with roofs, driveways and paving. Rain that once took hours to trickle into the street drain now arrives in minutes, and far more of it arrives. Multiply that across hundreds of developments in a catchment and the council’s pipes, channels and creeks can’t cope. The result is more frequent and more severe flooding downstream.
Councils can’t realistically dig up and upgrade every drainage line each time someone builds a duplex. Instead, they require each new development to manage its own peak flows. That’s OSD: your site holds back the surge, then releases it at a controlled rate the existing network can handle.
The two numbers that define every OSD system: PSD and SSR
Almost every council OSD requirement comes down to two values, and once you understand them the whole system makes sense.
Permissible Site Discharge (PSD)
PSD is the maximum rate at which stormwater is allowed to leave your site, usually expressed in litres per second. Think of it as a speed limit on your stormwater outlet. It’s typically set so your developed site discharges no faster than it did before development — or no faster than the downstream network can safely accept. The actual figure varies by council and catchment, so always check the current council DCP or OSD handbook rather than assuming a number.
Site Storage Requirement (SSR)
SSR is the volume of storage you must provide, usually expressed in cubic metres. If water is arriving at your tank faster than the PSD allows it to leave, the excess has to go somewhere — the SSR is the holding capacity that absorbs the difference during the design storm. Again, the method for calculating it differs between councils, so the governing DCP or handbook is your starting point.
In practice: rain falls, runoff from your roof and paving drains to a tank or basin, a small outlet (often an orifice plate) restricts outflow to the PSD, and the storage volume fills and then slowly empties after the storm passes.
What an OSD system physically looks like
Most residential OSD systems in Sydney are one of the following:
- Below-ground tanks — concrete tanks under driveways or landscaped areas, common on tight lots.
- Above-ground basins — landscaped depressions in a lawn or garden that pond briefly during heavy rain.
- Pipe or chamber storage — oversized pipes or modular units providing storage in trench form.
Every system includes a discharge control pit with an orifice plate sized to the PSD, a trash screen to stop leaves and debris blocking the orifice, and an overflow path for storms bigger than the design event.
Who designs and approves it
OSD design is hydraulic engineering — it must be designed and certified by a suitably qualified civil or stormwater engineer. The design is assessed by council (or a private certifier against council requirements) as part of your DA or CDC. Calculations follow recognised methods consistent with Australian Rainfall & Runoff and the relevant council handbook, and pipework is designed to AS/NZS 3500.3. Many western Sydney councils base their requirements on the Upper Parramatta River Catchment Trust (UPRCT) On-Site Stormwater Detention Handbook — a topic worth its own article.
What builders should take away
- OSD is about rate, not volume — slowing water down, not eliminating it.
- PSD = how fast water may leave; SSR = how much you must store. Both come from your council’s current DCP or handbook.
- Get the OSD design sorted early — tank location affects your slab, driveway, landscaping and levels.
- Expect certification obligations at the end of the job, including a works-as-executed survey and often a positive covenant on title.
Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us for a fast, compliant OSD design for your next project.