Engineering Certifications & Reports
Stormwater engineering that passes council first time
Every development in NSW — from a residential granny flat to a multi-unit subdivision — generates stormwater runoff that must be managed on-site before it enters the council drainage network. The requirement for stormwater management is not optional: it is a condition of development consent in virtually every local government area, and failure to provide an adequate stormwater engineering solution is one of the most common reasons for council rejection or delayed approval.
IA Engineering provides complete stormwater engineering packages — from initial hydraulic assessment and drainage design through to On-Site Detention (OSD) sizing, BASIX compliance calculations, and the preparation of council-ready stormwater management plans. Our engineers understand the specific requirements of councils across Greater Sydney, the Hunter, and regional NSW, and we design solutions that pass first time.
Whether you are a homeowner adding a secondary dwelling, a developer subdividing land, or a builder trying to clear a stormwater condition on a Development Consent, our team provides technically rigorous stormwater solutions delivered on a timeline that keeps your project moving.
Stormwater engineering that passes council first time
- On-Site Detention (OSD) design
- Stormwater management plans
- BASIX drainage compliance
- Council & certifier submissions
Stormwater engineering services we provide
Our stormwater engineering scope covers the full design and certification lifecycle — from site hydrology through to council submission and construction-stage certification.
On-Site Detention (OSD) Design
On-Site Detention systems are required by most NSW councils for any development that increases impervious area. We calculate pre- and post-development runoff using the rational method or DRAINS software, size the detention volume for the design ARI event (typically 100 years), and design the storage system — whether a below-ground tank, above-ground basin, or roof storage — including orifice sizing and overflow discharge arrangements. Our OSD design packages are prepared to council engineering specification requirements.
Stormwater Management Plans
A Stormwater Management Plan (SMP) documents how stormwater from a development will be captured, detained, and discharged in accordance with council requirements. We prepare SMPs that address the full stormwater hierarchy — source control, lot retention, OSD, and connection to the council system — along with site plans, drainage diagrams, and engineering calculations. Our SMPs are structured to meet the submission requirements of Sydney Water, local councils, and private certifiers.
BASIX Stormwater Compliance
BASIX (Building Sustainability Index) requires all new residential development in NSW to meet water commitments — including rainwater tank collection, reuse for toilet flushing and garden irrigation, and stormwater management. We prepare BASIX-compliant stormwater designs and provide the supporting engineering documentation required by assessors and certifiers. Where a BASIX Certificate specifies particular drainage or water reuse measures, we design the engineering solution that delivers the commitment.
Drainage Infrastructure Design
Beyond OSD and management plans, we design the complete drainage infrastructure for a site — pipe sizing and invert levels, pit locations and types, easement documentation, connection to council mains, and drainage catchment delineation. For subdivisions and multi-lot developments, we prepare engineering drawings and specifications for drainage works that are required to be built to council standards before subdivision registration.
“Stormwater engineering is not glamorous, but getting it wrong means flooding in the downstream catchment, council rejection, and expensive retrofit work after the slab has been poured.”
— IA Engineering Design Team
OSD Design
100yr ARI
BASIX Compliant
All residential
All NSW Councils
Council submissions
Understanding OSD, BASIX, and council submissions
What is On-Site Detention and why do councils require it?
On-Site Detention (OSD) is a stormwater management strategy that temporarily stores runoff generated by a development during a storm event, and releases it at a controlled rate into the council drainage system. The goal is to ensure that the downstream drainage network — designed for a pre-development level of impervious area — is not overloaded by the increased runoff from new hard surfaces like roofs, driveways, and paving.
OSD detention volume is calculated by comparing the pre-development peak runoff rate against the post-development rate for design storm events — typically the 10-year and 100-year Average Recurrence Intervals (ARI). The difference in flow between the two scenarios determines the volume of water that must be temporarily held on site. The orifice plate or flow control device is sized to limit the discharge rate to the pre-development level, ensuring the downstream system is no worse off than before the development.
BASIX stormwater requirements explained
BASIX requires all new residential dwellings and significant alterations in NSW to achieve a water score — a measure of how efficiently the dwelling manages potable water consumption and stormwater. For stormwater, BASIX typically requires a minimum rainwater tank size (3,000 L or larger, depending on the dwelling), connected to toilet flushing and outdoor irrigation at minimum.
The BASIX Certificate specifies what must be installed. The engineering challenge is ensuring that the tank, pump, overflow, and connection to the stormwater system are designed correctly — sized to capture the required roof catchment area, discharge safely during large storm events, and meet the plumbing connection requirements of AS/NZS 3500.
Council vs private certifier submissions — what is the difference?
In NSW, development applications (DAs) are assessed by council, but Construction Certificates (CCs) can be issued by either council or an accredited private building certifier. The stormwater engineering requirements are set by the council through the DA conditions, but the CC certifier is responsible for confirming that the engineering documentation satisfies those conditions before construction commences.
Private certifiers generally have more flexible submission processes and can turn around reviews faster than council. However, the engineering standard required is identical — the drainage design must comply with the council's engineering specifications and the conditions of the DA consent, regardless of who issues the CC. IA Engineering's stormwater packages are prepared to satisfy both council and private certifier requirements, minimising the risk of requests for additional information.
When is a stormwater engineer required?
Most councils in NSW require a stormwater management plan prepared by a qualified engineer for any development that involves new impervious areas, changes to existing drainage, or where the site is in a flood-affected zone. This typically includes new dwellings, granny flats, secondary dwellings, dual occupancies, multi-dwelling housing, commercial buildings, and any subdivision. If your development consent has a condition requiring stormwater management documentation, you need an engineer — and the sooner you engage one, the fewer surprises you will encounter.
Standards & policy frameworks our designs comply with
How we work with you
We work directly with homeowners, builders, architects, town planners, and project managers to deliver stormwater engineering that fits into the broader project timeline. If you have a DA condition to satisfy, we review the conditions and provide a clear scope of what engineering documentation is required to discharge them. If you are at the design stage, we can provide preliminary advice on OSD requirements before the DA is lodged.
For straightforward residential projects, our stormwater packages are typically completed within five to ten business days of receiving site information. For complex sites, subdivisions, or commercial developments, we provide a project-specific scope and timeline at the outset so there are no surprises.
We have experience with the specific requirements of councils across Greater Sydney — including the Inner West, Canterbury Bankstown, Northern Beaches, Parramatta, Cumberland, Blacktown, The Hills, Georges River, Bayside, and Sutherland — as well as regional councils in the Hunter, Illawarra, and Central Coast. If your council has specific engineering specification requirements, we know them.
Drainage Engineering That Passes First Time