Hornsby Shire Council Stormwater Requirements for Builders

Development in the Bushland Shire

Hornsby Shire markets itself as the Bushland Shire for good reason – much of the LGA is national park, reserve and bushland gully. Development pressure is concentrated along the rail and road corridors, with steady activity in Hornsby, Asquith, Waitara, Thornleigh, Cherrybrook and the rural-residential areas around Galston and Dural. Hornsby Town Centre and the Asquith to Mount Colah corridor have seen apartment growth, while dual occupancies and granny flats are common across the established suburbs.

Because so many sites back onto bushland or drain to sensitive creeks, Hornsby Shire Council takes a careful approach to stormwater. Builders who understand the requirements before lodging save weeks of assessment time.

The Planning Framework: Hornsby DCP 2013

The key document is the Hornsby Development Control Plan 2013, which contains Council’s water management controls alongside the Hornsby Local Environmental Plan 2013. Council supports the DCP with engineering documents – including its Civil Works Specification and Water Sensitive Urban Design (WSUD) Reference Guidelines – that set out how drainage and treatment systems must actually be designed and built.

The intent running through these documents is consistent: development should not increase peak flows or pollutant loads reaching the Shire’s creeks and bushland, and stormwater systems must be durable and maintainable.

What Is Typically Required

• On-site detention (OSD): Hornsby generally requires OSD for subdivisions, dual occupancies and multi-dwelling development, and for non-residential projects involving external works. Council’s approach limits the permissible site discharge to around the pre-development flow from the site, with detention storage holding back the excess. Single dwellings can also need OSD where a covenant or local drainage constraint applies.
• Stormwater drainage plans: A drainage plan prepared in accordance with Council’s specifications normally accompanies the DA, showing the collection system, OSD, overflow paths and the point of discharge.
• WSUD and water quality: Larger residential and commercial projects are expected to incorporate treatment measures – rainwater tanks, bioretention, or proprietary devices – consistent with Council’s WSUD Reference Guidelines.
• Easements where gravity discharge fails: Many Hornsby blocks fall away from the street toward a gully. Where a site cannot drain to the kerb, an inter-allotment easement over downstream land, or another Council-accepted solution, is required.

Exact discharge limits, storm events and storage requirements should be confirmed against the current DCP and Council specifications at design time.

Local Factors: Gullies, Sandstone and Sensitive Creeks

Hornsby’s terrain is classic Hawkesbury sandstone country – ridgetop development falling steeply into bushland gullies that feed Berowra Creek, Cowan Creek and ultimately the Hawkesbury River. This creates several recurring design issues:

• Shallow rock: Sandstone close to the surface makes OSD tank and pipe excavation more expensive; early geotechnical awareness helps with realistic pricing.
• Dispersal to bushland: Uncontrolled discharge into bushland causes erosion and weed invasion, so Council looks closely at how rear-falling sites dispose of stormwater – level spreaders and dispersal trenches are assessed carefully and are not accepted everywhere.
• Sensitive receiving waters: Berowra Creek and its tributaries are valued waterways, which underpins Council’s emphasis on water quality treatment, erosion and sediment control during construction.

How Builders Should Prepare

• Establish the legal point of discharge first – on gully-backed blocks this single issue shapes the entire design and sometimes the feasibility of the project.
• Budget for rock excavation when locating OSD tanks on sandstone sites.
• Have the stormwater concept, driveway design and landscape plan prepared together to avoid clashes at CC stage.
• Prepare a proper erosion and sediment control plan – it is checked, not just filed, in this LGA.
• Confirm current numeric standards with Council or your engineer rather than reusing figures from old approvals.

Stormwater and OSD Design for Hornsby Shire

Contrive Consultants prepares stormwater drainage and OSD designs for dwellings, duplexes and townhouse projects across Hornsby Shire, from Asquith infill blocks to rural lots at Galston. See our civil and stormwater services or contact us today – call +61 497 848 111 or email info@contriveconsultants.com.au.