Stormwater Drainage Design Sydney

Stormwater drainage design is the engineering behind every pit, pipe, grate and overland flow path that carries rain off your site without flooding your building, your neighbour or the street. In Sydney, no development application involving new roof area, driveways or hardstand gets far without a competent stormwater concept, and no Construction Certificate is issued without detailed drainage plans that a certifier can stamp. Contrive Consultants prepares both – hydrologic and hydraulic calculations, pit and pipe layouts, on-site detention where required, and the documentation councils actually ask for.

We work from Rouse Hill in Sydney’s north-west and design stormwater systems across Greater Sydney and regional NSW: dual occupancies in Blacktown, townhouse sites in The Hills, industrial lots in Western Sydney’s employment precincts, subdivisions in growth-area release lands and everything between. Whether you are an owner lodging a DA, a builder chasing a CC, or a developer coordinating a full consultant team, our drainage designs are prepared by senior civil engineers who understand what each council’s engineering department expects to see – because that is what gets plans approved first time.

When council requires it

Almost every consent authority in Sydney triggers a stormwater requirement the moment impervious area changes. Typical triggers include:

Development applications (DA) – most councils require a stormwater concept plan or stormwater management plan with the DA for new dwellings, dual occupancies, multi-dwelling housing, commercial and industrial development, and subdivision.
Construction Certificates (CC) – detailed stormwater drainage plans with pipe sizes, invert levels, pit schedules and OSD details are a standard CC condition, and certifiers will not release the CC without them.
Complying Development Certificates (CDC) – CDC projects under the Codes SEPP still need drainage designed to a lawful point of discharge, and private certifiers routinely request engineer-prepared plans, particularly where an interallotment easement or OSD applies.
Difficult sites – low-side lots draining away from the street, sites relying on drainage easements over neighbouring land, and sites within overland flow paths all attract closer council scrutiny and need drainage design resolved early, ideally before the DA is lodged.

Each council’s Development Control Plan sets its own thresholds, minor/major storm standards and OSD policy, so we always confirm the specific requirements of your consent authority at the outset.

What we deliver

Stormwater concept plans for DA lodgement, coordinated with the architectural site plan
Detailed pit and pipe drainage layouts with longitudinal sections, invert levels and pit schedules for CC or CDC
DRAINS hydrologic and hydraulic modelling for minor and major storm events
On-site detention (OSD) sizing, tank details and orifice/weir outlet design where council policy requires it
Roof water and surface drainage design to a lawful point of discharge, including charged systems and rainwater tank connections
Interallotment drainage and easement design, with plans suitable for 88B instrument documentation
Overland flow path assessment and freeboard checks for habitable floor levels
Design certification, responses to council RFIs, and works-as-executed review at completion

Software & standards

Our drainage designs are modelled and documented against the references certifiers and council engineers check them against. Hydrology and pipe hydraulics are analysed in DRAINS using procedures from Australian Rainfall and Runoff 2019 (ARR 2019), including current design rainfall data. Property drainage components – roof gutters, downpipes, surface drains and site pipework – are designed to AS/NZS 3500.3:2021 Stormwater drainage. Where water quality treatment applies, we model treatment trains in MUSIC, including MUSIC-link submissions for participating councils. Road-related drainage follows Austroads guidance, and every design is checked against the relevant council DCP and engineering specification, since minor/major storm standards and OSD rules differ between local government areas.

Our process

Brief and site review – we confirm your approval pathway (DA, CC or CDC), review survey, architectural plans and council policy, and identify the lawful point of discharge.
Concept design – we resolve the drainage strategy: gravity or charged, direct street discharge, easement or OSD, and flag any constraints that could affect the architectural layout while changes are still cheap.
Detailed design and modelling – DRAINS modelling, pipe and pit sizing, OSD calculations and preparation of full drainage drawings.
Documentation and lodgement support – issued plans and supporting calculations formatted for your certifier or council, with prompt responses to any RFI.
Construction support and certification – clarifications during construction, review of works-as-executed survey and engineering certification for the occupation certificate stage.

Why Contrive

Approvals-focused – we design to the standards your specific council and certifier assess against, which keeps RFIs short and approvals moving.
One coordinated group – through the Contrive group we align drainage with ARQI‘s architecture, URBA‘s planning advice, Sweptpaths‘ traffic work and partner surveyors, so your consultants aren’t contradicting each other.
Sydney-wide and NSW-wide – from Rouse Hill we service every Greater Sydney council and take on regional NSW projects.
Senior-led design – your drainage is designed and checked by experienced civil engineers, not delegated and forgotten.

Frequently asked questions

Do I need a stormwater drainage plan for a DA in Sydney?

In most cases, yes. If your proposal adds roof area or paving – a new home, duplex, granny flat with significant hardstand, commercial building or subdivision – the council will generally require at least a stormwater concept plan with the DA. Some smaller works can defer detail to the CC stage, but the discharge strategy still has to be shown to work. We confirm the exact requirement with your council before starting.

What is a lawful point of discharge?

It is the location where your site’s stormwater can legally leave the property – typically the street gutter, a council drainage pit, or an interallotment easement draining through downhill neighbours. Low-side lots that fall away from the street often cannot reach the street by gravity, which is where easements, charged systems or pump-out arrangements come in. Resolving this early is the single most important step in any drainage design.

Can stormwater plans be done for CDC projects?

Yes. Complying development still has to drain lawfully, and private certifiers commonly require engineer-designed drainage plans before issuing the CDC – particularly where OSD, easements or charged lines are involved. We prepare CDC drainage documentation regularly and liaise directly with your certifier.

How much does stormwater drainage design cost?

It depends on site difficulty, council requirements and whether OSD, easements or water quality modelling are involved – a straightforward duplex is a very different exercise to an industrial estate. Send us your survey and plans and we will give you a fixed fee proposal for the scope your project actually needs.

Related services

Ready to get your drainage design moving? Call Contrive Consultants on +61 497 848 111, email info@contriveconsultants.com.au or contact us online for a fixed-fee proposal on your Sydney or NSW project.