When Do You Need OSD in Sydney? Triggers by Development Type
The question every builder asks at quoting stage
“Do we need OSD on this one?” It matters, because an on-site detention system can add real money to a build and real space pressure to a site layout. The honest answer is always the same: it depends on the council and the specifics of the development — every Sydney council sets its own triggers in its DCP or OSD handbook, and they differ. But the patterns are consistent enough that you can usually predict the answer before you lodge. Here’s how the common development types tend to play out.
What councils are actually looking at
OSD triggers almost always come back to one thing: how much extra impervious area the development creates, and whether the catchment downstream can absorb the faster runoff. Councils typically frame their triggers around development type, site area, the amount of new roof and paving, or a combination. Some councils apply OSD across their whole LGA; others only in nominated catchments with known drainage capacity problems. Always check the current DCP for the specific lot — and remember the trigger can differ between a DA assessed by council and a CDC assessed against the same council’s requirements.
New single dwellings
A new house on a vacant lot is commonly caught by OSD requirements, particularly in western Sydney councils where catchment-wide detention policies apply. The logic: a new dwelling converts a largely pervious lot into roof and driveway. On knock-down rebuilds, some councils give credit for the impervious area that lawfully existed before demolition, which can reduce — but rarely eliminate — the requirement. Don’t assume; the credit rules vary.
Duplexes and dual occupancies
Duplexes are caught by OSD requirements in most Sydney councils, and it’s easy to see why: two dwellings, two driveways, and typically a high proportion of the lot covered in hard surface. If you’re pricing duplex sites, it’s safest to carry an OSD allowance from day one and treat any exemption as a bonus. On tight duplex lots, the OSD tank location also interacts with the driveway and the inter-allotment drainage, so get the stormwater concept resolved before the architecture is finalised.
Granny flats and secondary dwellings
This is the grey zone. A granny flat adds modest roof area, and councils differ widely: some exempt secondary dwellings below a size threshold, some require detention whenever impervious area increases beyond a set amount, and some assess it case by case based on the downstream system. Complying development approvals add another layer, since the certifier still has to apply the relevant council’s stormwater requirements. The cost of getting this wrong is a redesign after the slab is planned — so confirm the trigger with the council’s current DCP before quoting.
Townhouses, terraces and residential flat buildings
Multi-dwelling housing is almost universally caught. Higher site coverage, common driveways and basement structures mean these projects generally need engineered OSD — often a below-ground tank integrated with the basement or driveway design, with the storage and permissible site discharge (PSD) calculated to the council’s handbook. On these jobs OSD isn’t a line item; it’s a design driver that belongs in the earliest site layout discussions.
Commercial and industrial development
Warehouses, retail and commercial buildings create large roof and hardstand areas, so OSD is a near certainty on greenfield sites and most significant redevelopments. The storage volumes are bigger, but so are the options — car park ponding, landscaped basins and large underground tanks all come into play. Change-of-use and minor alterations may escape the trigger if impervious area doesn’t increase, but additions and hardstand expansions usually don’t.
Alterations, additions and pools
Small additions often fall under council thresholds, but cumulative impervious area matters: a big alteration plus a new pool surround plus an extended driveway can tip a site over the trigger. Some councils assess the total impervious area of the lot as it will exist after the works, not just the new bit.
The practical takeaway for builders
- Check the trigger before you quote, not after DA lodgement. The council’s current DCP or OSD handbook is the source of truth.
- Duplexes, multi-dwelling and commercial: assume OSD is required.
- Granny flats and additions: genuinely council-specific — verify every time.
- Where OSD applies, the PSD and site storage requirement (SSR) come from the council’s adopted method — get an engineer to run the numbers early so the tank has somewhere sensible to go.
Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW. Check our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us for a quick assessment of whether your next site needs detention.