Knock-Down Rebuild Stormwater in 2026: What Certifiers Check

The knock-down rebuild is how much of established Sydney renews itself. In suburbs from Ryde to Revesby, Castle Hill to Concord, the land is worth more than the house on it, and owners who like their street but not their floor plan demolish and start again. For builders, KDRs are steady, repeatable work. For stormwater, they carry a trap that catches someone on almost every project: the assumption that because the old house drained fine for sixty years, the new one will too.

It will not — at least not in the eyes of the council or the certifier. The moment you demolish and lodge for a new dwelling, you are designing to current standards, not the standards of 1962. The old cottage almost certainly predates on-site stormwater detention entirely. Its downpipes may run to an absorption pit that silted up during the Whitlam years, or straight through the neighbour’s yard under a gentleman’s agreement nobody alive remembers making. None of that carries over. The new dwelling needs a lawful point of discharge, compliant roof drainage, and — in many councils — an OSD system the original house never had. This post covers what has genuinely changed in KDR stormwater practice heading into 2026, where the surprises hide, and the checklist certifiers now work through at CC and OC stage.

Why the Old House Is No Guide to the New One

Three things reset when you knock down and rebuild.

First, the approval trigger. A new dwelling — whether via DA or complying development — is assessed against the council’s current DCP and engineering specification. Many Sydney councils require OSD for new dwellings or major additions even where the demolished house was exempt, because the exemption belonged to the era, not the land. Triggers vary council to council, so the first task on any KDR is confirming whether detention applies under that council’s current rules. Guessing costs you an RFI at best and a redesign at worst.

Second, the impervious area. A 1950s cottage with a single carport becomes a two-storey home with double garage, alfresco, wider driveway and often a pool. Even where footprints look similar, total impervious area typically jumps — and impervious area is what drives detention requirements.

Third, the documentation standard. The old house was never asked to prove where its water went. The new one must show a lawful point of discharge on an approved plan, and everything built must match that plan closely enough for an engineer to certify and a certifier to accept.

What Has Genuinely Changed in Recent Years

Plenty of noise circulates about “new stormwater rules”. Most of it is exaggerated, but several real shifts have accumulated, and together they explain why a KDR lodged in 2026 gets more stormwater scrutiny than one lodged five years ago.

ARR 2019 Is Now Simply How It Is Done

Australian Rainfall and Runoff 2019 — with its 2016 BOM IFD data, ensembles of ten temporal patterns per storm duration from the ARR Data Hub, critical duration testing and pre-burst considerations — has fully displaced the old single-design-storm habits in NSW drainage practice. Council development engineers expect DRAINS models built this way and increasingly reject shortcuts. For a single-dwelling KDR the modelling effort is modest, but it must be done properly: the days of a hand calculation on a template being waved through are largely over in the larger Sydney councils.

The 2024 Climate Chapter Is Filtering into Council Specs

In late August 2024, ARR’s updated climate change guidance (Book 1 Chapter 6) was published, aligned with IPCC AR6. It works from SSP scenarios and applies climate factors by scenario and time horizon, with proportionally higher uplifts for short-duration storms — exactly the storms that govern residential OSD and pipe design. Through 2025 and into 2026, councils have progressively updated engineering specifications to reference it, and reviewers increasingly expect climate-adjusted rainfall in flood and drainage assessments. Adoption is not uniform, but the direction is one-way. Practically, this can mean slightly larger detention volumes and pipes than the same house needed a few years ago — ask your engineer to confirm the council’s current position rather than assuming last project’s settings still apply.

Certifier Scrutiny Has Tightened — Noticeably

The biggest change for builders is not in the rainfall data; it is at the certification desk. Private certifiers, under sustained regulatory attention across the NSW building industry, have become far less willing to sign off stormwater on thin paperwork. In practice that means closer checking that the constructed drainage matches the approved plans, insistence on a proper works-as-executed (WAE) survey rather than a builder’s sketch, engineer’s OSD certification with supporting evidence, and confirmation that the positive covenant for OSD maintenance is actually registered on title before the Occupation Certificate is issued. Jobs that would have scraped through in the past now stall at OC stage over missing WAE surveys. Treat the certification trail as part of the build, not an afterthought.

BASIX Tanks and Council Detention Still Have to Be Reconciled

Every new KDR dwelling carries BASIX commitments, almost always including a rainwater tank plumbed for reuse. The tank and the council’s detention requirement are separate obligations that share a site: a BASIX tank sized for reuse does not automatically satisfy OSD, and an OSD tank does not satisfy BASIX. Some councils allow combined tanks with a dedicated detention zone above the reuse volume; others want the systems kept separate. Coordinating the two — volumes, levels, overflow routing — is standard engineering work, but only if someone does it before the slab is poured. Mismatched BASIX and stormwater documentation is a recurring certifier query.

Demolition: Your One Chance to Learn the Truth

Here is a piece of advice that costs nothing and saves projects: use the demolition stage to find out where the site’s water actually goes. Once the old house is down and before the new works begin, the ground tells you things no dial-before-you-dig search or 1960s sewer diagram will.

Common discoveries on Sydney KDR sites include:

Undocumented pipes heading in directions that make no sense until you realise they cross the rear boundary into the neighbour’s yard — a historic arrangement with no easement behind it, which cannot be relied on for the new dwelling.
Absorption pits and rubble trenches that served the old roof, long past capacity, and generally unacceptable to councils on Sydney’s clay soils today.
No street connection at all — the discovery that the block never had a piped point of discharge, and the kerb is higher than the rear of the lot.
Shared or combined drains from an era of looser plumbing, sometimes tangled with the sewer.

Each of these changes the stormwater strategy for the rebuild. If the block falls away from the street, the options are an inter-allotment drainage easement over a downhill neighbour, a charged (sealed) line where the levels genuinely allow water to rise back to the kerb, or a pump-out system where the council accepts one. Easements involve a neighbour’s consent and legal work; pumps involve council conditions and a homeowner who must understand they own a pump for life. Neither is a disaster — but both are far cheaper to resolve at design stage than at frame stage. Physically verify the point of discharge as soon as demolition opens the ground.

When Does a KDR Trigger OSD?

Council-dependent — and that is the honest answer. Broad patterns across Greater Sydney:

Many councils require OSD for any new dwelling, on the logic that a rebuild is new development assessed under current controls.
Others apply an impervious-area threshold, so a modest single-storey rebuild might scrape under while the typical two-storey with pool and wide driveway does not.
Councils in the north-west that reference the UPRCT On-Site Stormwater Detention Handbook will generate a PSD and SSR for the site under that method; others use their own permissible discharge rates or pre/post-development matching across a range of AEPs.
A minority of catchments with regional or trunk detention exempt individual lots.

The design outcome for a KDR is usually a modest below-ground tank or a landscaped above-ground storage, an orifice plate in a discharge control pit, a screen, an overflow path, and a positive covenant on title requiring the owner to maintain the system. On tight lots the tank ends up under the driveway with a trafficable lid. None of this is exotic; all of it needs to be on the approved plans before the CC, because retro-fitting detention around a house already under construction is miserable work.

Roof drainage, meanwhile, is designed to AS/NZS 3500.3:2021 — gutter capacities, downpipe numbers and positions, and overflow provisions. Certifiers do look at this now, particularly overflow provisions on box gutters.

The 2026 Certifier Checklist for KDR Stormwater

Based on what private certifiers across Sydney are consistently asking for, here is the practical checklist to build into your program.

Before the Construction Certificate

Detailed stormwater plan consistent with the DA consent or CDC requirements, showing the lawful point of discharge.
OSD design and calculations to the council’s nominated method, with a DRAINS model where required — increasingly with the council’s expected climate-adjusted rainfall settings.
BASIX certificate commitments (tank size, reuse connections) consistent with the stormwater and hydraulic plans.
Any required inter-allotment easement created or clearly secured, where the discharge strategy relies on one.
Erosion and sediment control plan for demolition and construction.

Before the Occupation Certificate

Engineer’s OSD certification confirming the system was built in accordance with the approved design.
WAE survey by a registered surveyor showing as-built tank volume, levels, orifice size and pit details.
Positive covenant for OSD maintenance registered on title.
Rainwater tank installed and plumbed per BASIX commitments.
Site drainage complete and consistent with the approved plan — including the bits behind the garage the landscaper wanted to change.

The recurring OC-stage failures are the last-metre items: the orifice plate never installed, the tank poured smaller than designed, the covenant paperwork never lodged. Have your engineer inspect the OSD before it is backfilled and buried under the driveway. Photographs of an open tank with a tape measure in frame have rescued more OCs than any other single document.

Get the Sequence Right

The KDR projects that sail through are the ones where the stormwater engineer is engaged at concept stage, alongside the building designer — not after the DA drawings are finished. A concept-stage stormwater review on a KDR is quick: confirm the point of discharge, check flood and overland flow mapping, establish whether OSD is triggered and roughly how big it needs to be, and flag any easement or pump risk while the floor plan can still move. That one step protects the design fee, the DA timeline and the handover date. The alternative is designing the house first and asking the water to fit in afterwards — which, on the wrong block, it simply will not.

If you’re yet to lock in a builder for your rebuild, 13 Homes works alongside our engineers on knock-down-rebuild projects across Sydney.

Frequently Asked Questions

The old house never had OSD — surely the new one is exempt?

No. The exemption belonged to the era the old house was approved in, not to the land. A knock-down rebuild is assessed as new development under the council’s current controls, and many Sydney councils require OSD for new dwellings even where the demolished house predated detention entirely. Whether it is triggered on your lot depends on the specific council’s rules — confirm early.

Can my BASIX rainwater tank double as the OSD tank?

Sometimes, partially. Some councils accept a combined tank with a dedicated detention volume sitting above the BASIX reuse volume, with the orifice controlling the detention zone. Others require separate systems. The reuse volume itself generally cannot be counted as detention because it may be full when the storm arrives. It is a design question for your engineer and a policy question for the council — resolve both before CC.

What if demolition reveals my block drains through the neighbour’s yard?

An old pipe with no easement behind it is not a lawful point of discharge for the new dwelling. Your options are negotiating a formal inter-allotment drainage easement with the downhill neighbour, a charged line back to the street where levels allow, or a council-accepted pump-out system. Each has different cost and timeline implications, which is exactly why the point of discharge should be verified before the design is locked.

What is a WAE survey and why is the certifier insisting on it?

A works-as-executed survey is a surveyor’s record of what was actually built — tank dimensions and volume, invert levels, orifice size, pit locations — overlaid against the approved design. Certifiers rely on it, together with the engineer’s OSD certification, as evidence the detention system works as approved. Expect it to be non-negotiable at OC stage in 2026.

Planning a knock-down rebuild? Contrive Consultants handles KDR stormwater across Greater Sydney and NSW — point-of-discharge investigations, OSD design and DRAINS modelling to current council requirements, and the certification and WAE documentation your certifier will ask for at OC. Explore our stormwater engineering services or get in touch before your design is locked in — call +61 497 848 111 or email info@contriveconsultants.com.au.