What is On-Site Detention (OSD) and when does a Sydney council require it?
On-Site Detention (OSD) is a stormwater system that temporarily stores rainwater runoff on your property and releases it slowly, so your development doesn't increase peak flows downstream. Sydney councils require OSD when a development increases impervious area (roofs, paving, driveways) enough to worsen runoff — triggers and storage volumes are set in each council's Development Control Plan (DCP).
On-Site Detention (OSD) is a stormwater system that temporarily stores rainwater runoff on your property and releases it slowly, so your development doesn't increase peak flows downstream. Sydney councils require OSD when a development increases impervious area (roofs, paving, driveways) enough to worsen runoff — triggers and storage volumes are set in each council's Development Control Plan (DCP).
How OSD works
When you add roof, paving and driveway to a site, rainwater that used to soak into the ground now runs off faster and in greater volume. OSD captures that extra runoff in a tank, basin or below-ground storage and meters it out through a small orifice (the discharge control pit) at a controlled rate. The storage stays empty between storms and only fills during rain, then drains down. Two numbers govern every OSD design:
- PSD (Permissible Site Discharge) — the maximum rate, in litres per second, your site is allowed to release to the council system. It is set so the downstream pipes and channels are not overloaded.
- SSR (Site Storage Requirement) — the volume of storage, usually in cubic metres, needed to hold the difference between developed runoff and the PSD across the design storms.
The engineer sizes the storage so that, across all storm durations up to the council's design event (commonly the 1% AEP / 100-year event, with the 20% AEP / 5-year event also checked), the site never discharges faster than the PSD.
When does a Sydney council require OSD?
There is no single statewide trigger — OSD is a council requirement set in each Local Government Area's DCP and stormwater/WSUD policy. In practice, OSD is required when a development materially increases impervious area or runoff and the downstream drainage cannot absorb the extra flow. Common triggers across Greater Sydney include:
- New dwellings, dual occupancies, townhouses and unit blocks on lots that drain to a council or Sydney Water stormwater system.
- Additions and alterations that increase impervious area beyond a set threshold. For example, in the Upper Parramatta River catchment, single dwellings, extensions and additions can trigger OSD where the increase in impervious area exceeds 150 m² on flood-affected land.
- Commercial, industrial and subdivision development, where increased hardstand is significant.
- Sites where council mapping shows downstream drainage capacity is constrained or the land is flood-prone.
Some councils exempt minor works, sites that drain directly to a creek or harbour (charged systems), or developments below a stated impervious-area increase — but you cannot assume an exemption. The controlling document is always the relevant council's DCP stormwater chapter, read together with its OSD or WSUD policy. Many western Sydney councils — including those historically within the
Upper Parramatta River Catchment Trust (UPRCT) area such as Parramatta, Blacktown, The Hills and Cumberland (former Holroyd) — apply the UPRCT *On-site Stormwater Detention Handbook* method, which standardises a Site Storage Requirement of around
470 m³ per hectare and a Permissible Site Discharge of around
80 L/s per hectare for the catchment. Eastern and southern councils (for example City of Sydney, Bayside, Georges River, Sutherland Shire, Inner West) set their own PSD and storage parameters in their own specifications.
Standards and regulations that apply
A compliant OSD and stormwater design in NSW is built on:
- AS/NZS 3500.3 Plumbing and drainage — Stormwater drainage — the core standard for roof, surface and subsoil drainage design, sizing and connection. The 2021 edition has been superseded by the 2025 edition (work commenced on or after 20 October 2025 must comply with the 2025 version); confirm the edition your council and certifier require.
- Australian Rainfall and Runoff (ARR 2019) — Geoscience Australia's national guideline for design rainfall and flood estimation, used with current IFD (Intensity-Frequency-Duration) data to size detention storage.
- National Construction Code (NCC) 2022, Volume Three (Plumbing Code of Australia) — governs plumbing and drainage work and adopts the AS/NZS 3500 series as Deemed-to-Satisfy provisions.
- The relevant council DCP, stormwater/drainage specification and WSUD policy — these set the PSD, SSR, design storm, orifice and pit details, and submission requirements for your LGA.
- State Environmental Planning Policy (Exempt and Complying Development Codes) 2008 — relevant where works are assessed as exempt or complying development rather than via a full DA.
OSD also interacts with
BASIX and
Water Sensitive Urban Design: a rainwater tank sized for reuse can often offset part or all of the OSD storage, but the reuse volume is assessed separately from your BASIX commitment. The right design balances detention, reuse and council rules in one stormwater concept and Stormwater Management Plan submitted with your DA.
What OSD costs and what drives the price
OSD design fees are scoped to the site, not a flat rate. The honest cost drivers are:
- Site size and increase in impervious area — bigger catchments need more storage and more modelling.
- Council and method — a UPRCT-method LGA is more standardised than a council requiring full ARR storage routing.
- Storage type — above-ground tanks/basins are cheaper to design and build than below-ground tanks, pipe storage or proprietary modular systems on tight or sloping lots.
- Flood and overland flow constraints — flood-prone or landlocked-drainage sites need extra analysis.
- Documentation depth — concept only, or full design, Stormwater Management Plan, calculations and certification for DA and Construction Certificate.
We don't publish dollar figures because they'd be misleading across these variables. For a firm number on your site,
get a fast quote and we'll price the exact scope.
Frequently Asked Questions
❯ Do I always need OSD for a new house in Sydney?
No — it depends on your council's DCP and your site. Many new dwellings and additions do trigger OSD, but some sites are exempt (for example where they drain directly to a creek or the harbour, or where the increase in impervious area is below the council's threshold). We check your specific LGA's rules before designing.
❯ What is the difference between OSD and a rainwater tank?
A rainwater tank stores water for reuse (irrigation, toilet, laundry) and is usually a BASIX measure. OSD is detention — it stays empty between storms and exists only to slow runoff. A correctly sized reuse tank can offset OSD storage in many councils, but the two serve different purposes and are assessed separately.
❯ Can a rainwater tank reduce or remove my OSD requirement?
Often yes. Several Sydney councils allow an oversized reuse tank to offset OSD, with policies that may require, for example, several times the detention volume provided as usable reuse storage. The exact ratio and eligibility are set by your council — we design the combined system to satisfy both.
❯ How big does the OSD tank need to be?
It is calculated, not guessed. In UPRCT-method areas the Site Storage Requirement is around 470 m³ per hectare of site, scaled to your area and impervious increase; other councils set their own storage rate. The final volume comes from the PSD, your developed runoff, and the council's design storm under ARR 2019.
❯ When in the project do I need the OSD designed?
Before lodging your Development Application. Councils require a stormwater concept and, in most cases, an OSD/Stormwater Management Plan as part of the DA, then detailed design and certification at Construction Certificate stage.
Reviewed by Youssef Emad, MIEAust 5372671, Registered Professional Engineer (NSW) PRE0002581, RPEQ 37639, Metric Engineering. NSW-based structural and civil engineering — design and certification.