What Is Onsite Detention and Do You Need It for Your New Build in NSW?
- 7 days ago
- 2 min read
If you're building or developing in NSW, there's a decent chance your council will ask for an onsite detention system - and if you've never heard of it before, you're not alone. It's one of those things that comes up mid-project and throws people off. So here's the plain-English version.
What Actually Is Onsite Detention?
Onsite detention - or OSD - is basically a way of temporarily holding stormwater on your property before releasing it slowly into the council drainage system. Think of it like a buffer. When it rains hard, instead of all that water rushing straight off your site and overloading the street drains, an OSD system captures it and lets it trickle out at a controlled rate.
It can be an underground tank, a surface basin, or part of a larger stormwater management setup. The size and design depends on your site, your council's requirements, and how much hard surface (roof, driveway, paving) is on the property.
When Is OSD Actually Required in NSW?
This is where it varies. Each council has its own rules, but generally you're looking at OSD being required when:
You're building a new home or dual occupancy
You're doing a significant extension or adding a second storey
The new impervious area on your site exceeds a certain threshold
The council's DCP (Development Control Plan) specifically calls it out
Some councils in Greater Sydney, the Central Coast and the Hunter are pretty strict about this. Others have specific exemptions for smaller residential builds. It's worth checking early - not after your DA gets kicked back.
How Is an OSD System Designed?
A civil engineer will look at the catchment area of your site, the council's allowable discharge rate, and the rainfall data for your area. From there, they'll calculate the required storage volume and specify what type of OSD system works for your block.
The design gets documented in a stormwater management plan, which goes in with your DA or CC. Councils want to see the numbers - not just a rough sketch.
Can a Builder or Plumber Handle This?
A licensed plumber can install an OSD tank once it's been designed and specified. But the actual design and hydraulic calculations? That needs a civil engineer. Councils won't accept a stormwater plan without engineering backing, and getting it wrong can mean redesigns, delays, or failed inspections down the track.
It's not about being overly cautious - it's just how the approval process works in NSW.
What Happens If You Skip It?
If OSD is required on your site and you don't include it in your application, your DA will likely get held up or knocked back. And if it gets picked up during construction, you're looking at rectification costs that are way more painful than just getting the design done properly upfront.
Need an OSD Design for Your NSW Project?
At Elevate Engineering Consultants, we handle onsite detention design for residential and small commercial projects across the Central Coast, Sydney, and the Hunter. We work with builders and homeowners to get the documentation right so your project keeps moving. Get in touch if you need a hand.