OSD for Knock-Down Rebuilds & Tight Sydney Infill Lots

Big OSD requirements, small lots

Sydney’s housing growth is increasingly happening on land that’s already developed: knock-down rebuilds, duplexes replacing single dwellings, and infill projects on lots that were subdivided generations ago. These sites come with a stormwater squeeze — councils still require on-site detention, but the lot offers nowhere obvious to put it. A 450 m² block with a two-storey duplex, double driveways and minimum landscaping doesn’t have a spare corner for a basin. Here’s how OSD actually gets solved on tight sites.

First, check what credit the existing development earns

On a knock-down rebuild, the site wasn’t pervious before you arrived — there was a house, a driveway, maybe a big slab of paving. Many councils account for lawfully existing impervious area when assessing detention requirements, on the logic that the drainage network already receives that runoff. Depending on the council’s method, this can reduce the storage you need.

Two cautions. First, the rules differ sharply between councils — some credit existing impervious area generously, others assess the developed site from scratch; only the current DCP or OSD handbook answers it. Second, evidence matters: a survey of the existing development before demolition is far stronger than an aerial photo argument afterwards. If a credit might help, capture the evidence while the old house is still standing.

Where the tank goes when there’s nowhere for the tank

On tight lots, OSD placement is a process of elimination. The realistic candidates:

Under the driveway

The default for knock-down rebuilds and duplexes. A below-ground concrete tank with a trafficable lid uses space the development needed anyway. Watch the clashes: driveway grades, the sewer connection, NBN/services trenches and street tree roots all compete for the same corridor. Coordination beats improvisation.

Under landscaping — carefully

Tanks under the front setback or courtyard work, but check the council’s deep soil and landscaped area definitions: some councils don’t count landscaping over structures toward minimum landscaped area. Losing a compliance point on landscaping to win one on stormwater is a bad trade.

Pipe or chamber storage along the side boundary

Where volumes are modest, oversized pipes or modular storage in the side setback trench can deliver the storage in a strip a tank can’t use. Maintain clearances to footings and the neighbour’s boundary, and keep inspection access workable.

Combined detention/retention tanks

Most new dwellings need a BASIX rainwater tank anyway. A combined tank — reuse storage below, dedicated detention air space above the orifice — solves two requirements in one excavation and is often the single biggest space-saver on a small lot, where the council accepts the arrangement.

Levels: the constraint that outranks everything

Storage only works if it drains, and tight lots are where gravity gets difficult. The tank must sit low enough to collect roof and surface runoff, yet high enough to discharge to the lawful point of discharge — usually the street gutter or an inter-allotment easement. On low-side lots that fall away from the street, this single issue can dominate the design: rear drainage easements, charged (sealed) roofwater lines to the tank, or pump-out systems for below-invert areas may enter the conversation, each with its own council rules. Resolve the discharge point and the levels before the floor levels are fixed, not after.

Design moves that buy back volume

• High early discharge (HED). An HED outlet arrangement lets the system discharge at near the permissible site discharge from early in the storm, which reduces the storage volume required — exactly what a tight site needs.
• Use the geometry you have. A long shallow tank under a driveway can deliver the same volume as a deep cube that would have hit rock or groundwater.
• Don’t over-pave. Every square metre of permeable surface or reduced hardstand trims the runoff the system must detain. Permeable paving credits vary by council — ask before relying on them.
• Split storage. Two smaller storages (e.g. a small tank plus shallow surface ponding on a paved area, where permitted) can fit where one big tank can’t.

The sequencing lesson

Every painful tight-lot OSD story has the same plot: stormwater was left until the architecture, the driveway and the levels were locked, and the tank became an afterthought with nowhere to go. The fix costs nothing: get a stormwater concept — discharge point, tank location, indicative volume per the council’s current requirements — at sketch design stage. On small lots, OSD isn’t a detail to be fitted in later; it’s one of the constraints the design grows around.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW, with particular experience squeezing compliant detention onto knock-down rebuild and infill lots. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us today.