Servicing due diligence means confirming, before you buy, that a site can lawfully drain its stormwater and connect to sewer at the yield you are paying for. The most common failure in Sydney is the no-discharge-point problem: the block falls away from the street, so roof and paved water cannot reach the kerb by gravity, and there is no drainage easement over the downhill neighbour. On sewer, the question is where the Sydney Water or Hunter Water main sits, whether the building will be over or near it, and whether a Section 73 certificate will be needed. Neither problem is unsolvable, but both can cost more than the extra dwelling earns.
Key takeaways
- Every new dwelling needs a lawful point of stormwater discharge, which in practice means the kerb, a council pipe or pit, or an inter-allotment drainage easement.
- A lot that falls to the rear usually needs an easement over a neighbour, and Section 88K of the Conveyancing Act 1919 is the court route if they refuse.
- Most Sydney councils require on-site detention for dual occupancies and multi dwelling housing, and the tank takes site area.
- Sydney Water’s building over or adjacent to assets rules can shift a dwelling footprint, and subdivision needs a Section 73 certificate.
- Outside the Sydney Water and Hunter Water areas, the local council is the water and sewer authority and its own connection rules apply.
The rule underneath all of it
Councils will not approve a dwelling that discharges concentrated stormwater onto a neighbour. AS/NZS 3500.3:2021 sets the plumbing standard and the council DCP sets the drainage policy, but the principle is older than both: water must go to a lawful point of discharge. For a lot that slopes to the street, that is the kerb and gutter through a kerb outlet, or a council pit. For a lot that slopes away from the street, there are only a few options.
Reading the fall before you buy
A detail survey with spot levels is the only reliable way to know which way a site drains. Short of that, the clues are the direction the existing house’s downpipes run, the location of the neighbours’ outlets in the kerb, and the contours on the council’s flood or drainage mapping. In the older suburbs of Blacktown, Fairfield and Cumberland, streets were often built along ridges, so a good share of lots fall to the rear. In the newer estates of Marsden Park, Schofields, Austral and Leppington the subdivision engineering usually provides a drainage easement or a rear lot connection, and the Section 88B instrument will show it.
The options when the block falls the wrong way
| Option | When it works | Who has to agree |
|---|---|---|
| Inter-allotment drainage easement | A downhill neighbour has a pipe or kerb outlet you can reach | The neighbour, with a registered easement and compensation |
| Existing easement on title | The 88B already benefits your lot | Nobody, but check the pipe exists and has capacity |
| Council pipe or pit at the rear | A drainage reserve or trunk line adjoins the lot | The council, through a connection approval |
| Charged or pumped system | Small additions or single dwellings in some councils only | The council, and many refuse it for new dwellings |
| Absorption or infiltration | Sandy soils and low run-off, rare on Western Sydney clays | The council, with a geotechnical report |
The easement route is the usual answer, and it is a negotiation rather than a design task. The downhill owner is entitled to say no. If they do, Section 88K of the Conveyancing Act 1919 lets the Supreme Court impose an easement where it is reasonably necessary and the neighbour is compensated, but that is a slow and expensive path and a poor basis for a feasibility. Price the easement as a real cost and a real delay, and do not exchange on a rear-falling lot without a plan.
On-site detention and the space it takes
Most Sydney councils require on-site detention (OSD) once a site’s impervious area increases beyond a single house, so a dual occupancy or a set of townhouses will almost always need one. Blacktown, Penrith, Liverpool, Camden and Campbelltown each publish their own OSD requirements, and the storage volume depends on the catchment and the council’s site storage rate. The practical point for due diligence is that an OSD tank or basin takes site area, sits at the low point, and needs an overflow path to the same lawful discharge point. Our civil colleagues at Contrive Consultants size the OSD at concept stage so the planning layout does not have to change later.
Sewer: where the main is and what it costs
In Greater Sydney, the Illawarra and the Blue Mountains the sewer authority is Sydney Water, and in the Lower Hunter it is Hunter Water. Three questions decide the sewer side of servicing due diligence.
- Where is the main? Sydney Water’s sewer service diagram shows the main and the existing connection. Mains often run along rear boundaries in older suburbs, exactly where a second dwelling wants to go.
- Will the building be over or near the asset? Building over or adjacent to a sewer main triggers Sydney Water’s approval process, and the requirements can include piers, concrete encasement or a changed footprint. The rules are set out on Sydney Water’s developing and building page.
- Is a Section 73 certificate needed? A Section 73 Compliance Certificate under the Sydney Water Act 1994 is required for subdivision and for many developments that add dwellings. It is obtained through a Water Servicing Coordinator, and it may come with a requirement to extend or amplify the main at the developer’s cost.
Hunter Water applies its own version of the same process across Newcastle, Lake Macquarie, Maitland, Cessnock and Port Stephens, with details on the Hunter Water site. In regional NSW, the council is usually the water and sewer authority and a Section 305 certificate under the Water Management Act 2000 replaces the Section 73 process.
Growth area sites and the missing main
In parts of the North West and South West Growth Areas the lead-in sewer has not yet reached every street. A site zoned for housing with no sewer within reach is a servicing problem, not a planning one, and the answer can be a long wait or a very expensive extension. Ask Sydney Water for the servicing status of the parcel and read the precinct’s servicing strategy before you value the land on its zoning alone. Trunk drainage easements and riparian corridors in these precincts also take land and appear on the precinct plan rather than the LEP.
What we ask for on a servicing check
- A detail survey with levels, or at minimum the fall direction from a site visit.
- The sewer service diagram and the water main location.
- The Section 88B instrument, to find any drainage easement, positive covenant for OSD, or sewer easement.
- The council’s OSD policy and site storage rate.
- The Section 10.7(5) advice, which sometimes notes overland flow paths or known drainage constraints.
Those five items let us say, before exchange, if the stormwater solution is a kerb outlet, an easement negotiation or a redesign. That answer belongs in every development due diligence report and it is often the difference between a duplex that works and one that does not.
Frequently asked questions
What is a lawful point of discharge for stormwater in NSW?
It is a place the council accepts stormwater from your lot, normally the kerb and gutter, a council drainage pit or pipe, or a registered inter-allotment drainage easement leading to one of those. Discharging onto a neighbour’s land without an easement is not lawful.
Can I pump stormwater to the street if my block slopes to the rear?
Some councils accept a charged or pumped system for minor works or a single dwelling, but most Sydney councils refuse it for new dual occupancies and townhouses because pumps fail. Assume you will need a gravity easement unless the council’s drainage policy says otherwise.
Do I need a Sydney Water Section 73 certificate for a duplex?
You will need one if the duplex is subdivided, and often where the development adds a dwelling. The certificate confirms the water and sewer servicing is adequate and can carry works or contribution requirements. It is applied for through a Water Servicing Coordinator.
Does on-site detention apply to a granny flat?
It depends on the council. Many councils exempt a secondary dwelling under a set impervious area, while others require OSD once the total impervious area on the lot passes a threshold. Check the council’s drainage policy before the layout is set.
URBA’s planners run servicing due diligence with civil input on every duplex, townhouse and subdivision site we assess, so the stormwater and sewer answer is known before design fees are spent. See our dual occupancy and duplex planning service, or request a fee proposal for a site check in Blacktown or anywhere across Sydney.