On-Site Detention (OSD) Design
Councils cap the amount of stormwater your site may discharge. OSD holds the difference back. An oversized OSD is expensive to build — we size it to the requirement, not above it.
What you receive
OSD design drawings and details
Storage volume and orifice sizing calculations
Permissible Site Discharge and Site Storage Requirement compliance check
OSD certification documentation where council requires it
How we do it
Sizing to your council's DCP method, or the Upper Parramatta River Catchment Trust method where it applies
Modelling in DRAINS (Watercom) to ARR 2019 rainfall or an older version where required.
Above-ground, below-ground, tank, pipe and surface storage options priced against each other
Orifice plate and discharge control design
Integrated detention and retention systems where the site suits it
Surface detention worked into driveways, courtyards and parking where topography allows — often the cheapest solution
When you need it
When your council DCP triggers OSD, usually above a set impervious area or development threshold, or where a consent condition requires it. Common on new builds, dual occupancy, multi-dwelling, subdivision, commercial and industrial sites, and on larger alterations and additions. Complying development can also trigger OSD under the Codes SEPP.
Related services Stormwater Management Plans · Hydrology Assessment · Water Sensitive Urban Design
Frequently Asked Questions
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Typically 2–4 weeks from receipt of architectural plans and survey.
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Usually surface or driveway storage, then above-ground tanks, then buried tanks. Site fall dictates what's possible
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Yes, in a dual-purpose tank, where the detention volume stays available.
Contact us
Send us your site address, and we'll come back with a fixed fee, usually within one business day