OSD Design in Parramatta: The Honest Guide That Saves You Time, Money, and Headaches
OSD design in Parramatta is one of those requirements that lands on people’s desks at the worst possible moment — usually when they are already deep into the DA process and suddenly realise their drainage documentation is incomplete. I have had homeowners call us after their certifier has flagged missing OSD calculations two weeks before their expected construction certificate date. That kind of delay is stressful and completely avoidable. So let me explain what OSD actually is, why Parramatta City Council takes it seriously, and what a properly prepared OSD design looks like — before any of that happens to you.
What Is OSD and Why Does Parramatta City Council Require It?
OSD stands for on-site detention. It is a temporary stormwater storage system — almost always an underground tank — that collects the excess rainwater runoff your development generates and releases it slowly into the council drainage network, rather than sending it all rushing out at once.
Here is why that matters in Parramatta specifically.
Every time a property gets developed, more hard surface gets added. A new roof. A concrete driveway. A paved courtyard. All of that surface is now impervious — rain hits it and runs straight off, instead of soaking into the ground. Multiply that across thousands of new dwellings in one of the fastest-growing LGAs in NSW, and without some form of detention, the council’s existing pipes and channels would be overwhelmed every time a serious storm rolled through western Sydney.
OSD design in Parramatta is the council’s way of making each development responsible for managing its own excess runoff. Your tank absorbs the surge, holds it, and lets it out slowly once the peak of the storm has passed. The downstream drainage network stays within capacity. Your neighbours do not get flooded. Everyone benefits — including you, because a properly designed OSD system protects your own property from backwater flooding too.
What Makes Parramatta’s OSD Parameters Different from Other Councils?
This is where a lot of generic drainage designs fall apart, and it is worth understanding clearly.
Parramatta City Council’s Development Control Plan sets out specific OSD design parameters — numbers that are particular to this LGA and are not simply interchangeable with what Blacktown, Hills Shire, or Cumberland Councils use next door. The two critical figures are:
Permissible Site Discharge (PSD): This is the maximum flow rate — measured in litres per second — that your site is allowed to release into the council drainage system during a storm event. It is calculated based on your site area using Parramatta’s specific PSD table. A smaller site has a lower PSD. A larger site gets a higher allowance. The point is that the figure is site-specific and must be taken from Parramatta’s own DCP, not estimated or borrowed from another council’s standards.
Detention Volume: This is the size of the tank needed to hold the difference between how much runoff your developed site produces and how much the PSD allows to leave. We calculate this using DRAINS hydraulic modelling software for the 1-in-100-year storm event — the worst credible scenario Parramatta Council uses as the design benchmark.
Use the wrong PSD figures and your detention volume will be undersized. Submit an undersized OSD design to Parramatta, and the council will reject it — or worse, it gets through but fails during an actual storm event. Neither outcome is good.
At RI Engineering, Parramatta’s current DCP parameters are always the starting point — not a shortcut, not an approximation.
What Does an OSD Design Parramatta Actually Include?
A proper OSD design Parramatta submission is not just a tank size scribbled on a drawing. What we deliver — and what Parramatta City Council and your certifier need to see — includes:
DRAINS hydraulic model showing pre-development and post-development runoff for the site, demonstrating the required detention volume for the 1-in-100-year event
Engineering drawings showing the tank dimensions, inlet and outlet configuration, location on site, overflow routing, and connection to the broader stormwater drainage system
Outlet structure design — the mechanism that controls the release rate. For most Parramatta residential sites, this is a slotted pit or orifice plate. It needs to be engineered correctly, because an outlet that silts up or is the wrong size defeats the entire purpose of the tank
Overflow routing — showing what happens when the OSD tank reaches capacity during an extreme event. Water still needs somewhere to go safely without causing damage to your property or your neighbours’
Specification notes for the builder and plumber — material requirements, installation sequence, and inspection requirements before backfilling
That last point matters more than people realise. An OSD tank that is installed incorrectly — wrong depth, wrong outlet position, wrong inlet connection — will not perform as designed. The documentation needs to be detailed enough that the tradesperson on site can install it correctly without guessing.
Which Parramatta Projects Trigger OSD Requirements?
The short answer is: most of them. Here is what consistently requires OSD design Parramatta in our experience:
New homes and knockdown-rebuilds — always. New roof area means new impervious surface, which means OSD.
Granny flats and secondary dwellings — yes. Even a modest granny flat with its own roof adds runoff to the system. Parramatta requires OSD as part of the CDC and DA stormwater documentation for these projects.
Duplexes and dual occupancies — yes, and the OSD calculation is more complex because you are dealing with two combined roof areas and a shared drainage system.
Townhouse and multi-unit developments — yes, and these require careful OSD design because the combined impervious area is significantly larger and the drainage infrastructure needs to function correctly for every dwelling in the complex.
Commercial developments — yes, and these often come with additional water quality requirements on top of OSD, depending on the project type and location.
Home extensions — depends on how much new hard surface the extension adds. If you are adding a ground-floor extension with a new roof section and extending the driveway, OSD recalculation is likely required. A purely internal renovation typically does not trigger it.
If you are unsure, the fastest thing to do is call us. We can tell you within minutes whether your project needs OSD based on your address and what you are planning to build.
Types of OSD Tanks Used in Parramatta
Underground polyethene tanks are the most common OSD solution for Parramatta residential sites. They are cost-effective, lightweight enough to install without heavy machinery in most residential lots, and available in a range of sizes. They sit beneath the driveway or rear yard and do not affect your usable outdoor space.
Precast concrete tanks are used where the required detention volume is large, where vehicle loading over the tank is heavy, or where the site conditions do not suit polyethene. More expensive to supply and install, but appropriate for the right application.
Tanked basement voids are sometimes used on constrained sites where there is simply no room for a conventional tank. The basement structure itself becomes the detention volume, with a controlled outlet. These require more detailed structural and hydraulic design coordination.
Aboveground detention basins are rare on residential Parramatta sites but are occasionally used on larger commercial or industrial developments where space permits and an underground solution is not cost-effective.
RI Engineering will recommend the right tank type for your specific Parramatta site — based on space, budget, soil conditions, and council requirements — and explain the trade-offs clearly before committing to a design direction.
Parramatta Clients Ask Us About OSD Design
Q: My builder told me OSD is optional for a granny flat. Is that right?
It is not right. Parramatta City Council requires OSD documentation for secondary dwellings and granny flats that add a meaningful impervious area to a site. Whether it applies to your specific granny flat depends on site coverage and the existing drainage situation, but “optional” is not an accurate description of how Parramatta treats OSD. If your builder is telling you it is optional, ask them to show you the council’s written confirmation — and if they cannot, get independent engineering advice before proceeding.
Q: How is the OSD volume calculated — is it a standard formula?
No — and this is one of the most important things to understand. The OSD volume is calculated using DRAINS hydraulic modelling of your specific site, using Parramatta’s specific PSD parameters. It is not a simple formula or a rule of thumb. The result depends on your site area, the proportion of impervious surface, the catchment characteristics, and the storm event being designed for. Two adjacent properties with the same site area but different development footprints will get different OSD volumes.
Q: Where does the OSD tank go on a typical Parramatta block?
Most commonly under the driveway, which is usually the most practical location — the area is already hardened for vehicle loading, there is typically clear access for installation, and it does not eat into garden or usable yard space. Rear yard installation is the next most common option. RI Engineering will show the proposed tank location in the design drawings and explain the reasoning.
Ready to Get Your OSD Design Sorted?
If your Parramatta project needs OSD design — or if you are not sure whether it does — the best next step is a quick conversation with our team.
📞 Call RI Engineering: +61 0451 452 932 📧 Email: riengineering2155@gmail.com 🌐 Website: riengineering.com.au
We turn OSD designs around in two to three business days for most residential Parramatta projects. We use Parramatta City Council’s current DCP parameters every time. And we write documentation that certifiers and builders can actually work with — not something that needs three rounds of clarification before anyone can proceed.
If you have a deadline, tell us when you call. We will do what we can to work around it.
