Civil & stormwater
Stormwater design for a new home or extension: what is normally required?
Understand site drainage, legal points of discharge, detention, charged systems and the information needed for stormwater design.
Stormwater design is more than drawing pipes from downpipes to the street. It must respond to levels, the lawful discharge point, council requirements, roof and paved catchments, neighbouring properties and the capacity of the proposed system.
Information needed before design
A reliable design usually starts with the current architectural plans and a feature and level survey. Council drainage information, title restrictions, easements and planning or permit conditions may affect the solution.
- Roof plan and proposed impervious areas
- Existing and proposed ground levels
- Legal point of discharge or approved connection
- Council standards and development conditions
- Known easements, pits, pipes and overland-flow constraints
On-site detention and retention
Some authorities require on-site detention to limit the peak discharge from a development. Others may require rainwater reuse or retention. Tank volume, outlet control and overflow routing must be designed as a coordinated system rather than selected independently.
Gravity systems, charged systems and pump-out
Gravity drainage is generally preferred where levels permit. A charged system may be used in suitable circumstances, but its sealed pipework, hydraulic grade and maintenance implications need to be understood. Low areas such as basement ramps can require a pump-out system with appropriate storage, redundancy and emergency overflow considerations.
Overland flow and neighbouring property
A pipe system cannot be assumed to capture every extreme event. Finished levels and surface grading should preserve a safe overflow path and avoid concentrating runoff onto neighbouring land. This coordination is particularly important on sloping, constrained or infill sites.
Common questions
Frequently asked questions
Can stormwater be discharged to the kerb?
Only where the relevant authority permits it and the site levels and connection details are suitable. The approved legal point of discharge must be confirmed for the project.
Do all extensions require an OSD tank?
No. Requirements depend on the authority, development type, site area and increase in impervious area. Project-specific council rules and approval conditions govern the outcome.
This article provides general information, not project-specific engineering, architectural or legal advice. Requirements vary by site, building, authority and jurisdiction.