OSD Tank Options Compared: Concrete Tanks, Basins & Pipes

One requirement, several ways to build it

Once your council confirms you need on-site detention, the next question is practical: what form should the storage take? The council generally doesn’t care whether your site storage requirement (SSR) is met with a concrete tank, a grassed basin or a run of oversized pipes – it cares that the volume is there, the permissible site discharge (PSD) is controlled, and the system can be maintained. But for the builder, the choice has big implications for cost, program and how the rest of the site works.

Here’s how the three main options stack up.

Option 1: Below-ground concrete tanks

The workhorse of Sydney OSD. A reinforced concrete tank – cast in situ or precast – buried under the driveway, courtyard or front setback, with a discharge control pit and orifice at the outlet.

Pros

• Zero loss of usable land. The surface above can be trafficable – driveways and parking are the classic location.
• Works on flat sites and tight lots where there’s simply no room for a basin.
• Durable, with a long design life and well understood by every council assessor.

Cons

• The most expensive option per cubic metre – excavation, reinforcement, waterproofing, trafficable lids and access covers all add up.
• Needs confined-space access arrangements for maintenance.
• Excavation can clash with rock, groundwater, services or tree protection zones – investigate before you price it.

Best suited to

Duplexes, townhouses, knock-down rebuilds and commercial sites where land value is high and every square metre of surface counts.

Option 2: Above-ground basins and surface storage

A landscaped depression – typically in a front or rear lawn – that ponds water for a short period during heavy rain, draining out through a controlled outlet. Paved courtyards and parking areas can sometimes be designed to pond shallowly as well.

Pros

• The cheapest storage you can build. It’s mostly earthworks and turf.
• Easy to inspect and maintain – everything is visible, no confined spaces.
• Can double as lawn or garden 99% of the time; it only works as a basin during storms.

Cons

• Consumes surface area, which tight Sydney lots rarely have to spare.
• Councils limit ponding depths in areas people use, and ponding near dwellings, in steep terrain or over private courtyards may not be acceptable – check your council’s DCP and OSD handbook.
• Levels are everything: the basin must drain completely and overflow safely, which doesn’t suit every fall of land.

Best suited to

Larger lots, battle-axe blocks with generous rear yards, and industrial or commercial sites where a landscaped basin or car park storage fits the layout.

Option 3: Pipe and chamber storage

Storage in trench form – oversized stormwater pipes, box culverts or modular plastic chamber/crate systems laid underground and connected to a discharge control pit.

Pros

• Fits in linear spaces – along a side boundary or under a driveway strip – where a rectangular tank won’t.
• Often quicker to install than a cast in situ tank; components arrive ready to lay.
• Shallower excavation than a deep tank in some configurations.

Cons

• Pipes store less volume per metre of trench than a purpose-built tank – large SSRs need a lot of pipe.
• Modular crate systems vary in council acceptance, structural rating and maintainability – confirm your council’s position before committing.
• Access points for inspection and cleaning still need to be designed in.

Best suited to

Smaller storage volumes, narrow infill lots, and sites where the storage can run alongside other drainage works already being trenched.

How to choose: five questions we ask on every site

• How much volume? Small SSRs open up pipe storage; large ones usually mean a tank or basin.
• What’s the land worth? If surface area is precious, go underground and accept the cost.
• Where are the levels? The storage must drain by gravity to the lawful discharge point – falls dictate location more than preference does.
• What’s underground already? Rock, groundwater, sewer mains and tree roots can rule options out fast.
• Who maintains it? An owner-occupier mows a basin; a strata scheme needs simple, safe access to whatever you build.

Often the best answer is a hybrid – for example, a small below-ground tank doing the heavy lifting with shallow surface ponding making up the balance. The right combination falls out of the hydraulic calculations and the site constraints together, which is why the OSD concept should be settled before architectural plans are locked in.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW. Explore our services, call +61 497 848 111, email info@contriveconsultants.com.au or get in touch and we’ll recommend the most cost-effective OSD option for your site.