Why Councils Reject OSD Designs (and How to Avoid It)

The resubmission cycle that quietly eats your program

A rejected stormwater design rarely kills a project — it just bleeds it. A council request for information (RFI) on your OSD design typically means weeks of delay while drawings are revised, recalculated, relodged and reassessed. Two or three RFI cycles can push a DA out by months. The frustrating part is how predictable the reasons are: assessment engineers knock back the same issues over and over. Here are the ones we see most, and how to make sure your submission isn’t in the pile.

1. Using the wrong council’s numbers (or last year’s)

The most common failure is the most basic: calculating the permissible site discharge (PSD) and site storage requirement (SSR) using a methodology, rate or assumption from a different council — or a superseded version of the current council’s DCP or OSD handbook. Sydney councils inherited similar frameworks (many from the UPRCT On-Site Stormwater Detention Handbook lineage) but have diverged in the details. Assessment engineers check the basis of calculation first, and a wrong reference undermines confidence in everything that follows.

Avoid it: confirm the current adopted document for the specific LGA — and the specific catchment, where councils vary requirements by catchment — before any calculation starts.

2. Storage that doesn’t add up

Councils recalculate your volumes. Designs get rejected when the claimed storage includes volume below the orifice invert (which never drains and therefore isn’t detention), counts a rainwater reuse tank as detention storage, ignores the depth limits on surface ponding, or relies on tank geometry the drawings don’t actually show. If the WAE survey would catch it later, the assessor will try to catch it now.

Avoid it: show the storage calculation transparently — stage-storage data, the datum, the orifice level — so the assessor can verify rather than question.

3. No believable overflow path

Every OSD system needs a safe route for storms bigger than the design event. Submissions fail when the overland flow path runs through the neighbour’s property, ponds against the dwelling, relies on flow uphill, or simply isn’t shown. Assessors are particularly alert to this on tight infill lots where levels are unforgiving.

Avoid it: show the surcharge route on the plan with levels proving it works — from the discharge control pit to the street or lawful discharge point, clear of buildings.

4. Discharge control details missing or generic

A surprising number of submissions show a tank but go vague at the business end: no orifice size, no plate level, no trash screen detail, a copied “typical” pit detail that doesn’t match the site levels, or a high early discharge (HED) arrangement mentioned in calculations but absent from the drawings. The discharge control pit is the part the council most cares about, because it’s the part that controls the flow.

Avoid it: a dimensioned discharge control pit detail — orifice diameter and invert, screen, weir, access — consistent between the calculations and the drawings.

5. No lawful point of discharge

The whole system must drain to a lawful discharge point — street gutter, council pit, drainage easement or inter-allotment system. Designs get refused when they discharge to a rear lot with no easement, assume an easement that was never created, or ignore that the lot falls away from the street. This isn’t an OSD detail; it’s a threshold question, and on low-side lots it can reshape the whole project.

Avoid it: resolve the discharge point first — title search for easements, survey for levels — before designing anything upstream of it.

6. Stormwater drawn after the architecture was frozen

Many rejections trace back to sequencing: the architect finalised the site layout, then the stormwater engineer was asked to “fit OSD in”. The result is tanks under landscaping that conflicts with deep soil requirements, basins in courtyards the DCP doesn’t permit, or pipes clashing with the basement. Councils reject the symptom, but the cause was the program.

Avoid it: get a stormwater concept — discharge point, tank location, levels — before the architectural plans are locked.

7. Sloppy documentation

Missing catchment plans, no design summary, inconsistent levels between drawings, calculations not aligned with Australian Rainfall & Runoff 2019 design rainfall, pipework not designed to AS/NZS 3500.3 — none of these alone may sink a design, but together they invite an RFI. Assessment engineers approve quickly what they can verify quickly.

The pattern behind all of it

Councils rarely reject OSD because the engineering problem is hard. They reject it because the submission used the wrong rulebook, hid the numbers, or skipped the details that matter. A complete, current, site-specific package — calculations to the council’s adopted method, real details, proven levels — typically goes through on the first pass.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW — built to the current DCP, documented for first-pass approval. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us today.