Do I need an engineer for an OSD tank under a driveway in NSW?

Why two engineers, not one

An OSD tank does two jobs at once, and each is governed by different rules.

The hydraulic job — sizing the detention

Your council sets two numbers in its Development Control Plan (DCP): the Permissible Site Discharge (PSD), the maximum rate (L/s) your site may release to the public system, and the Site Storage Requirement (SSR), the minimum volume the tank must hold. These are calculated from your impervious area, local rainfall intensity and the catchment's controls — for example, the Upper Parramatta River Catchment Trust (UPRCT) On-site Stormwater Detention Handbook, which underpins many Western Sydney council DCPs, has historically required on the order of 470 m³/ha of storage at a PSD of around 80 L/s per hectare. Calculations follow Australian Rainfall and Runoff (ARR) methods, and the connected pit-and-pipe design must comply with AS/NZS 3500.3:2025 (Plumbing and drainage — Stormwater drainage). Many councils restrict who may design and certify OSD to accredited practitioners — civil engineers (NER/CPEng), accredited hydraulic consultants, or accredited surveyors — so a generic builder's sketch will be rejected.

The structural job — carrying the driveway

The moment the tank sits under a trafficable surface, it becomes a loaded structure. The roof slab and walls must be designed to carry vehicle wheel loads, not just soil and water. The relevant standards are:

• AS 1170 series (AS/NZS 1170.0 / 1170.1) — structural design actions and imposed (live) loads, including vehicle and traffic actions on the slab.
• AS 5100 (Bridge design) — commonly referenced for the vehicle/wheel loading model where heavier vehicles (service trucks, fire appliances, delivery vehicles) can access the driveway.
• AS 3600 (Concrete structures) — design of the reinforced concrete slab, walls and base.
• AS 3735 (Concrete structures for retaining liquids) — supplements AS 3600 and governs crack control and watertightness for the tank as a water-retaining structure, so it does not leak into the driveway subgrade or neighbouring property.

If the tank is precast, AS 3850 (Prefabricated concrete elements) applies to the panels and their connections. Deep tanks (generally 1200 mm or deeper) also need access provisions such as step irons and a compliant access cover rated for the traffic above.

What council actually asks for

For an OSD under a driveway, a typical NSW council pathway requires:

1. A stormwater concept plan with the Development Application — drainage layout, tank location, overland flow path and connection point.
2. Detailed OSD design and calculations at Construction Certificate stage — PSD/SSR sizing, orifice details (councils commonly specify a minimum orifice plate, e.g. a 3 mm stainless plate with a minimum orifice diameter), pit and pipe sizing to AS/NZS 3500.3:2025.
3. A structural engineer's design and certificate for the tank as a trafficable, water-retaining structure — confirming the slab carries the design vehicle loads and the structure is watertight with no leakage through walls or under footings.
4. Work-as-executed (WAE) plans on completion to confirm the built tank matches the approved design.

Requirements vary between councils (Blacktown, Cumberland, Parramatta, Liverpool, Canterbury-Bankstown and others each publish their own OSD standard), so the design must be tied to your specific council's DCP and engineering specification. The National Construction Code (NCC) and the Environmental Planning and Assessment Act 1979 sit over the top, and where the work is class-1/class-2 building work it may engage Design and Building Practitioners Act 2020 registration for the engineering design.

When can you skip the OSD?

You generally can't decide that yourself — it's set by the consent. OSD is usually triggered when development increases impervious area or stormwater runoff above council thresholds. Some sites are exempt (for example where they drain directly to a trunk drainage system, a watercourse, or where council's controls specifically waive OSD), but that exemption has to be confirmed against the DCP, not assumed. Putting the tank under the driveway is often the practical choice on tight sites precisely because it frees up the rest of the block — but it always converts a simple tank into a structurally engineered one.

How Metric Engineering handles it

We design the OSD hydraulics to your council's PSD/SSR and AS/NZS 3500.3:2025, design the trafficable slab and tank to AS 1170, AS 3600 and AS 3735 (with AS 5100 vehicle loading where required), and issue the structural certification council needs — all in one coordinated package, so the hydraulic and structural sides actually match. We work across Greater Sydney and know the individual council OSD standards.

Frequently asked questions