15/08/2026
Council wouldn't approve this quadplex without it, and most people have never heard of it.
An OSD system, or On Site Detention system, temporarily holds rainwater runoff from roofs, driveways and hard surfaces during heavy storms and releases it slowly into the council drainage network.
Instead of all that water rushing off the property at once and overwhelming the street drainage system, it gets held on site and released gradually.
On a single home the stormwater volume is usually manageable. On a multi dwelling build, the volume is considerably higher.
That's why councils across Sydney require OSD systems on multi dwelling builds like this.
The part most people don't think about is the budget. If your build requires an OSD system and it hasn't been factored in upfront, it becomes an unexpected cost mid project that can impact the rest of your build budget.
Knowing whether your project needs one before your budget is finalised is something you’ll need to factor in.
Save this and share it with someone who’s in the planning stages of their build.