Your Practical Guide to Stormwater Planning

A new extension can look straightforward on a concept plan, yet the stormwater solution may determine where it sits, how much it costs and whether the proposal can be approved. This guide to stormwater planning explains the early decisions NSW homeowners, investors and small developers need to make before documentation is finalised.

Stormwater is the rainwater that lands on roofs, driveways, patios and other hard surfaces. Once a project adds roof area or paving, it changes how water moves across the site. The objective is not simply to get water away from the building. A suitable design must protect the home, avoid impacts on neighbouring land and meet the requirements of the relevant council or certifier.

Start stormwater planning before the design is fixed

Leaving drainage until the end of the design process often creates avoidable compromises. A proposed garage may occupy the only practical route to a lawful point of discharge. A new granny flat may leave insufficient room for on-site detention. A raised deck can change ground levels and direct runoff towards a boundary.

At the concept stage, establish the property's fall, existing drainage paths, street level, easements and available connection points. This is particularly important on sloping blocks, narrow sites, battle-axe allotments and properties affected by overland flow or flooding. The survey should show relevant levels, drainage structures and site features accurately. Assumptions based on a visual inspection can be expensive once detailed plans are prepared.

The proposed building footprint should then be tested alongside practical drainage requirements. This may influence finished floor levels, driveway grading, downpipe locations, landscaping and the position of tanks or detention systems. Good planning does not mean making the home smaller by default. It means resolving these constraints early enough to preserve the parts of the design that matter most.

Understand where stormwater can legally go

For many NSW residential projects, stormwater is discharged to the street kerb and gutter, a council drainage system, an approved inter-allotment drainage line or, in some cases, an on-site system. The correct option depends on the property and local controls.

A legal point of discharge is not the same as the lowest point on the block. Directing water onto a neighbour's land, into a private easement without authority, or towards a shared driveway can create disputes as well as compliance issues. Existing pipes also need to be investigated. Their presence does not necessarily mean they have adequate capacity, are in good condition or can be used for a new development.

If a connection to the street is proposed, council may require a specific kerb outlet detail and may control its location. Where a pipe crosses another parcel of land, formal drainage rights may be required. On sites with no convenient gravity connection, a pump system might be considered, but it introduces ongoing maintenance and reliance on power. It is generally better to achieve a gravity solution where the site allows it.

Detention, retention and infiltration are different

These terms are often grouped together, but they solve different problems. On-site detention, commonly called OSD, temporarily stores runoff and releases it at a controlled rate. It is frequently required where new works increase impervious area and could otherwise add pressure to the public drainage network.

Rainwater tanks retain water for reuse, such as toilet flushing, laundry supply or garden watering where permitted. They can support BASIX commitments and reduce potable water use, but a tank alone does not automatically meet OSD requirements. Its effectiveness for stormwater management depends on storage volume, outlet design, reuse demand and the applicable approval conditions.

Infiltration systems direct water into the ground through methods such as soakwells or trenches. They can suit some sites, but only where soil conditions, groundwater levels, slope, setbacks and site stability make them appropriate. They are not a universal answer for clay soils, heavily constrained urban lots or land with known drainage problems. A site-specific assessment is essential.

Check the council controls and approval pathway

Stormwater requirements vary across Sydney, the Central Coast and Newcastle. Councils may apply different standards for detention sizing, permissible discharge rates, water quality treatment, flood controls, connection details and documentation. Development Control Plans, engineering specifications and flood planning information can all affect the outcome.

For a Development Application, council may request a stormwater concept plan, calculations and detailed engineering as the proposal progresses. The required level of information depends on the scale and complexity of the works. A duplex, townhouse development or commercial fit-out involving significant external works will usually require more detailed coordination than a modest rear addition.

A Complying Development Certificate can be faster for eligible projects, but it does not remove the need to resolve drainage properly. The certifier must be satisfied that the development meets the relevant standards, and additional engineering documentation may still be needed. If a site is flood affected, subject to an easement, constrained by a drainage line or unable to achieve compliant discharge, a CDC may not be the right pathway.

This is why approval strategy should be considered alongside the design, rather than after plans have been drawn. The most efficient pathway is the one that fits both the proposed development and the site's constraints.

Design for the water that does not enter a pipe

Pipes, pits and tanks handle planned runoff. They do not always deal with intense rainfall, blocked grates or water flowing across the property from higher land. Overland flow needs its own consideration, especially in established suburbs where development may have altered historic drainage paths.

Finished floor levels should provide appropriate protection from expected flow paths. External surfaces need falls that direct water away from buildings, while drainage channels and grates must be accessible for cleaning. A driveway that falls back towards a garage may need a properly designed grated drain, but its discharge still has to be accommodated downstream.

Landscaping also has a role. Permeable areas, planted zones and sensible grading can reduce local ponding and improve how water moves through the site. However, landscaping cannot be relied upon to compensate for an undersized drainage system or an unlawful point of discharge.

Coordinate the right documentation early

A clear stormwater package typically starts with an accurate survey and is coordinated with the architectural plans. Depending on the project, it may include a hydraulic or civil engineering design, OSD calculations, drainage details, levels, pit schedules and connection information. For flood-prone land or complex sites, further advice may be necessary.

Coordination matters because small changes can have larger consequences. Moving a downpipe, changing a roof form, extending paving or raising a floor level can alter the drainage design. If these changes happen after engineering is complete, documents may need to be revised before approval or construction.

It is also worth clarifying who is responsible for each part of the work. Building designers prepare and coordinate the overall proposal, while suitably qualified engineers provide designs and certification where required. Contractors should build from approved documentation rather than making on-site changes to pipe sizes, falls or detention systems without advice. A system that is sound on paper still needs correct installation, inspection and future maintenance.

Common issues that increase cost or delay

The most frequent problems arise from incomplete site information and late decisions. An unrecorded easement, inadequate fall to the street, undersized existing drainage or a proposed structure over an access point can require significant redesign. So can an assumption that all roof water can simply be connected to an existing line.

Owners should also avoid treating stormwater as an invisible service that can be squeezed into leftover space. Detention tanks need access, pits require maintenance, and drainage routes must work around footings, landscaping and future alterations. The least visible solution is not always the most practical or economical over the life of the property.

For homeowners planning an extension or granny flat, the best time to ask stormwater questions is before committing to the layout. For investors and developers, early drainage feasibility can help confirm whether the intended yield is realistic. GAP Designers takes this approval-focused approach to help clients develop plans that respond to the site from the outset.

A well-considered drainage strategy gives a project more than a set of compliant pipes. It protects the building, respects neighbouring properties and gives the wider design a far better chance of progressing without unnecessary rework.