OSD Design Sydney

On-site detention (OSD) exists because Sydney’s street drainage was never sized for the city being built on top of it. When you replace lawn with roof and concrete, runoff leaves your site faster and in greater volume — so councils require you to detain the difference in a tank or basin and release it slowly, at no more than the rate the site discharged before development. Getting that right is a genuine hydraulic design task: storage volume, permissible site discharge, orifice sizing, weir overflows and freeboard all interact, and an OSD system that is sized by rule-of-thumb tends to fail either the council check or the site itself.

Contrive Consultants designs OSD systems for homes, duplexes, townhouses, apartment buildings and commercial and industrial sites across Greater Sydney and NSW. From our Rouse Hill office we work daily with the OSD policies of The Hills, Blacktown, Hornsby, Parramatta, Cumberland and dozens of other councils — each with its own method for setting permissible site discharge and site storage requirements. We take the system from first sizing through detailed tank and outlet design to the works-as-executed (WAE) certification council needs before releasing the occupation certificate.

When council requires it

DA and CDC triggers — most Sydney councils require OSD once a development increases impervious area beyond a threshold set in their DCP or stormwater policy. New dwellings on knock-down-rebuild lots, dual occupancies, multi-dwelling housing, and commercial or industrial buildings are the usual candidates.
Construction Certificate conditions — where OSD was accepted in concept at DA, the consent will condition detailed OSD design, calculations and structural details for approval before the CC is issued.
Catchment-based policies — some councils apply OSD in defined catchments where downstream trunk drainage is known to be at capacity, and exempt others; a handful accept alternatives where the site drains directly to a major watercourse.
Occupation certificate stage — consents almost universally require a WAE survey of the completed OSD system, checked and certified by an engineer, plus registration of a positive covenant or restriction on title for ongoing maintenance.

OSD rules are among the most council-specific requirements in NSW stormwater practice, so we always design against the current policy of your consent authority rather than a generic standard.

What we deliver

OSD sizing calculations establishing permissible site discharge (PSD) and site storage requirement (SSR) to the council’s adopted method
DRAINS modelling of the detention system across the full range of design storm durations to find the critical event
Tank layout and detail drawings — below-ground concrete tanks, above-ground basins, or storage within pipes and pits on tight sites
Outlet design: orifice plate sizing, discharge control pits, trash screens and emergency overflow weirs
High early discharge (HED) arrangements where the council permits combined OSD and rainwater reuse storage in a single tank
Integrated OSD–rainwater tank design coordinated with BASIX commitments, working alongside NathersPRO within the Contrive group
Structural coordination for trafficable tank lids, access grates and maintenance openings
WAE review and OSD certification for the occupation certificate, including checking the as-built survey against the approved design

Software & standards

Detention behaviour is modelled in DRAINS using ARR 2019 design rainfall, testing a full suite of storm durations because the critical event for storage is rarely the same as the critical event for pipe flow. Site pipework and overflow provisions are documented to AS/NZS 3500.3:2021. Where the OSD interacts with water quality devices, the treatment train is modelled in MUSIC (via MUSIC-link for participating councils). Above all, OSD design is governed by the individual council DCP or OSD handbook — PSD/SSR methodology, minimum orifice sizes, freeboard and covenant requirements all vary by LGA — and our designs cite and comply with the specific policy your assessor will check against. Road and driveway interfaces follow Austroads guidance where relevant.

Our process

Brief and policy check — we confirm whether OSD applies to your site, under which council policy, and whether any exemption or alternative pathway is realistically available.
Preliminary sizing — early PSD/SSR calculations so the architect can allow for the tank footprint and levels before the layout is locked in.
Detailed design — DRAINS modelling, tank and outlet detailing, orifice and weir sizing, and coordination with structural, landscape and BASIX requirements.
Approval documentation — calculation reports and drawings issued for council or certifier approval, with RFI responses handled promptly.
Construction to certification — site queries answered during the build, WAE survey reviewed, and the OSD certification issued so your occupation certificate is not held up at the last hurdle.

Why Contrive

Approvals-focused — we design to the checking method your council actually uses, which is the difference between one submission and three.
Whole-of-project coordination — architecture (ARQI), planning (URBA), BASIX/thermal (NathersPRO) and surveying partners sit within the Contrive group, so the OSD, rainwater tank and landscape design are resolved together, not patched later.
Greater Sydney and NSW coverage — we hold current working knowledge of OSD policies across Sydney’s councils and confirm requirements for any NSW authority we have not worked with recently.
Senior engineers on every job — sizing, outlet hydraulics and certification are handled by experienced civil engineers who stand behind their signatures.

Frequently asked questions

Can my rainwater tank count towards OSD storage?

Sometimes. Several Sydney councils allow a combined tank where the reuse volume sits below a high early discharge (HED) outlet and the detention volume above it, so the airspace needed for detention is always available regardless of how full the reuse portion is. Other councils insist on fully separate storages. We design both arrangements and will tell you which your council accepts before you commit to a tank size.

What are PSD and SSR in OSD design?

Permissible site discharge (PSD) is the maximum rate your site may release to the street or drainage system during the design storm; the site storage requirement (SSR) is the detention volume needed to hold back everything above that rate. Councils set the method for calculating both — some publish fixed rates per hectare for defined catchments, others require site-specific pre-development modelling — which is why the same duplex can need noticeably different tanks in different LGAs.

Why does the orifice size matter so much?

The orifice is the throttle that makes the whole system work: too large and the tank drains fast without ever detaining the storm; too small and it blocks with debris. Councils typically specify a minimum orifice diameter and require a trash screen, and where the calculated orifice falls below that minimum the design has to change — a larger storage, a different outlet arrangement or an HED configuration. This is exactly the kind of detail we resolve in design rather than discover during certification.

What is WAE certification and who provides it?

Works-as-executed (WAE) certification is the engineer’s confirmation, based on an as-built survey, that the constructed OSD matches the approved design — storage volume, orifice size, levels and overflow provisions. It is routinely a condition before the occupation certificate can be issued. We review the surveyor’s WAE plans, inspect where required, resolve any departures with the certifier and issue the certification.

Related services

Need OSD designed, approved and certified without hold-ups? Call Contrive Consultants on +61 497 848 111, email info@contriveconsultants.com.au or use our contact page to get a fee proposal for your site.