Sydney’s urban landscape is changing fast. With infill development, duplexes, and townhouse subdivisions becoming the norm, builders and homeowners often face smaller lots and tighter spaces. While this makes efficient use of land, it also introduces major stormwater challenges particularly during heavy rainfall events like the recent deluge that left parts of Sydney submerged.
We specialise in stormwater and hydraulic design for residential and duplex developments across Sydney. In this article, we explain why small sites are particularly tricky, common mistakes we see, and how professional design can save time, money, and headaches.
Why Small Sites Are a Stormwater Challenge
Smaller lots create unique drainage problems:
- Limited space for natural runoff: When the site area is small, there’s less room for water to disperse naturally. Every extra square metre of roof, driveway, or patio contributes to runoff.
- High impervious surface ratio: Most small sites are heavily paved or built over, which prevents water absorption into the soil.
- Complex council requirements: Councils often require On-Site Detention (OSD) systems, rainwater tanks, or stormwater pits to prevent flooding downstream.
Even minor rainfall events can overwhelm a poorly planned small site, leading to flooding, erosion, or damage to neighbouring properties.
Common Mistakes on Small Sites
- Ignoring natural contours: Not accounting for slopes and site levels can direct water toward buildings or neighbouring properties.
- Undersized drainage: Pipes, pits, and OSD tanks that are too small fail to cope with stormwater volume.
- Poor coordination with landscaping: Garden beds, retaining walls, and driveways need to be integrated into the stormwater design.
- Overlooking council rules: Many small-lot developments require detailed hydraulic calculations to gain approval.
The result? Rework, delays, and higher costs often after construction has started.
Smart Solutions for Small Sites
Designing effective stormwater systems on small lots requires both technical knowledge and practical creativity. Solutions include:
- On-Site Detention (OSD) tanks: Temporarily store water and release it slowly to reduce peak flows.
- Permeable paving: Allows water to soak into the ground rather than run off hard surfaces.
- Swales and shallow basins: Can be integrated into landscaping to store and slow water.
- Integrated drainage design: Coordinating roof, driveway, and yard drainage to maximise efficiency.
At Strive Engineering, we combine hydraulic calculations with site-specific design to create compliant, buildable stormwater systems, even on tight lots.
Case Example: Sutherland Duplex Development
A recent duplex project in Sutherland demonstrated the challenges of small-lot stormwater:
- Lot size: 650m²
- Roof area: 450m²
- Paved driveway and courtyard: 90m²
Without an OSD system, heavy rain caused overland flow onto the neighbouring property. Our solution included:
- A below-ground OSD tank sized to council specifications
- Permeable driveway paving
- Redirected roof runoff to the street via properly sized pipes
The result: a compliant, flood-safe site, no delays and a satisfied client.
Takeaways for Builders and Homeowners
- Plan early: Engage an engineer before construction to ensure stormwater solutions are integrated into the design.
- Don’t compromise on space: Even small sites can accommodate OSD or detention basins if planned properly.
- Consider long-term maintenance: Systems must be accessible for cleaning and inspection.
- Work with experienced engineers: Knowledge of council rules, local rainfall patterns, and site constraints is critical.
Final Thought
Small sites don’t have to mean big stormwater problems. With smart hydraulic design, proper planning, and practical implementation, even tight lots can safely manage runoff, protect property, and meet council compliance.
At Strive Engineering, our mission is to deliver practical, compliant, and cost-effective stormwater solutions for Sydney residential developments.
Contact Us
If you’re building a residential or duplex development on a small site and need professional stormwater or hydraulic engineering, contact us today:
📞 0424364267
📧 admin@striveengineering.com.au
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