Stormwater & OSD on a heritage site — commercial building, Ku-ring-gai
Council — Ku-ring-gai Council
Development — Alterations and additions to an existing heritage cottage, and construction of a new commercial building
Services — Stormwater Management Plan · Water Sensitive Urban Design · Hydrology Assessment · On-Site Detention
Year — 2024
Outcome — Approved
Background
The client wanted to add a new commercial building alongside an existing heritage cottage, on a site that was partially low-lying.
The problem
Adding the commercial building triggered the full set of controls at once — water sensitive urban design, on-site detention and rainwater reuse. Headroom across the site was slim, which ruled out the conventional answer of putting a tank underground.
Our solution
We split the site into two drainage systems rather than forcing one across both buildings — the commercial building and the heritage cottage have different constraints, and treating them as a single catchment would have compromised both.
On the commercial side, the work was finding the volume that satisfied three requirements simultaneously: detention sized to council's charts, rainwater reuse verified against council's stream flow requirements through a water balance calculation, and water quality targets met through a MUSIC model built around shallow-depth proprietary products chosen specifically for the available headroom. The decisive move was lifting the OSD system onto a pad. That single change gave us gravity drainage to the street on a low-lying site — no pump, no pump chamber, and nothing for the client to power or maintain for the life of the building.
On the heritage side, we ran a hydraulic analysis of the existing roof drainage to establish whether it had the capacity to convey water to kerb and gutter as it stood. It did, which meant the cottage's existing system could be retained rather than rebuilt — the outcome that matters most on a heritage structure, where every intervention is a consent risk. Cleanout pits and a dispersion trench pick up the remaining surface bypass.
The commercial building was delivered with a compliant stormwater, detention and water quality system inside the headroom available, and the heritage cottage was left substantially as it was.
Was it approved? Yes.
On-Site Dentention, Rainwater, WSUD detail