Stormwater Drainage Design for Commercial Developments in NSW

When You Need a Stormwater Plan

Most NSW councils require a stormwater management plan with your DA. Any development that adds roofs, paving, or hardstand changes how rain runs off the site. More hard surfaces means more runoff, faster. The council wants proof that your project will not flood neighbours or overload the public drainage system.

This applies to new commercial buildings, extensions, car parks, and warehouse developments. Even a change of use that adds impervious area can trigger the requirement. If you are lodging a DA in Sydney, check your council's Development Control Plan (DCP) for the stormwater section. It will tell you exactly what to submit.

The stormwater plan is prepared by a hydraulic engineer. It covers roof drainage, site drainage, on-site detention (if required), and the connection to the public stormwater system.

• Roof drainage: gutters, downpipes, and rainheads sized to AS/NZS 3500.3:2025. The 2025 edition introduced new overflow design rules for eaves gutters and valley gutters. (AS/NZS 3500.3:2025, Section 3)
• Site drainage: pits, pipes, and channels that collect surface water from car parks, driveways, and landscaped areas and move it to the outlet point
• On-site detention (OSD) if required by council: a tank or basin that holds stormwater and releases it at a controlled rate to limit peak discharge
• Connection to the public stormwater system: the pipe from your site to the council or Sydney Water stormwater main. Sydney Water requires a Section 73 compliance certificate for connections to their assets.
• Overland flow paths: the route water takes when a storm exceeds the pipe capacity. The design must show that overland flow does not enter buildings or cross property boundaries in an uncontrolled way.
• Stormwater quality treatment: for larger developments, councils may require MUSIC modelling and water sensitive urban design (WSUD) measures such as gross pollutant traps, bioretention basins, or proprietary filtration devices

OSD tanks or basins hold stormwater temporarily and release it slowly through a restricted outlet (usually an orifice plate). They stop your development from increasing peak flows into the public system. Most Sydney councils require OSD for any development that increases impervious area.

The tank usually sits underground, below the car park slab or in a landscaped area. Some sites use above-ground basins where space allows. The tank fills during heavy rain and drains down over 12 to 24 hours after the storm passes.

Size depends on three things: total site area, impervious percentage after development, and the council's permissible site discharge (PSD). PSD is the maximum rate your site can release stormwater, measured in litres per second. Typical PSD values for Sydney councils range from 20 to 80 L/s per hectare of site area.

A typical commercial OSD tank holds 5,000 to 20,000 litres. A large warehouse or retail site may need 30,000 litres or more. The Upper Parramatta River Catchment Trust (UPRCT) standard, used by many western Sydney councils, requires 470 m³ of storage per hectare with a PSD of 80 L/s per hectare.

Every OSD system needs an access hatch for maintenance. Councils require a positive covenant on the property title to make sure future owners maintain the tank. Sydney Water requires a formal maintenance agreement if the OSD discharges into their drainage system.

• Stormwater concept plan with the DA: shows drainage layout, OSD tank location, overland flow paths, and proposed connection point to the public system
• Detailed stormwater design with the CC: includes pipe sizes, pit locations, invert levels, OSD calculations, and full compliance with AS/NZS 3500.3:2025
• Council DCP stormwater requirements: each council has its own DCP chapter on stormwater. Requirements vary. Some councils specify OSD tank types, minimum pipe sizes, or pit spacing. Always check the specific DCP for your local government area.
• Sydney Water Section 73 compliance certificate: required if your site connects to a Sydney Water stormwater asset. Sydney Water checks that your development has adequate stormwater services and issues the certificate after inspection.
• MUSIC modelling for stormwater quality: required by many councils for larger commercial developments. MUSIC (Model for Urban Stormwater Improvement Conceptualisation) models pollutant removal. Council sets the pollutant reduction targets, typically 80% total suspended solids, 45% total phosphorus, and 45% total nitrogen.

Stormwater design is split across two project stages. The concept goes with the DA. The detailed design goes with the Construction Certificate (CC).

• Stormwater concept for DA: $2,000 to $5,000. Covers the drainage layout, OSD location, and connection strategy. Enough for council to assess the proposal.
• Detailed stormwater design for CC: $3,000 to $10,000. Includes pipe sizing calculations, pit schedules, OSD sizing, long-section drawings, and specification.
• OSD design and calculations: usually included in the detailed design fee. If separate, expect $1,000 to $3,000 depending on complexity.
• MUSIC modelling: $2,000 to $5,000 if required by council. Not all commercial developments need it.

Costs depend on site size, number of buildings, drainage complexity, and council requirements. A simple single-building commercial site sits at the low end. A multi-building development with OSD, MUSIC modelling, and multiple discharge points sits at the high end.