PERTHDrainage plumbing diagram: the map of your underground drains
The WA drainage plumbing diagram: what it records, why you need it before digging or landscaping, how it fits the 2024 certification, and AS/NZS 3500.2.
Somewhere in the records for most Perth properties is a diagram that shows where the underground drains run, and it is one of the most useful documents a homeowner can have. It is called a drainage plumbing diagram, and knowing it exists, and how to get it, can save you from digging into a sewer line you did not know was there.
What it is
A drainage plumbing diagram, sometimes called a DPD, is a record of the layout of the sanitary drainage on a property: where the drains run, where the connections and inspection points are, and where the system meets the main sewer. It is produced when drainage work is done and certified, and Building and Energy holds these diagrams as a reference that homeowners, licensed plumbers and others use to determine the drainage layout of a property.
Because the drainage is underground and invisible, the diagram is often the only way to know where the pipes are without excavating or running a camera. That makes it valuable at exactly the moments when guessing is expensive.
When you need one
Any time you are going to disturb the ground. Building an extension, laying a slab, installing a pool, putting in footings for a patio, or major landscaping, all of these risk hitting a drain if you do not know where it runs. A collapsed or cracked sewer line from an excavator is a costly and messy repair, and it is entirely avoidable if the diagram is consulted first.
It also matters when there is a drainage problem. If waste is backing up or a drain is running slowly, the diagram tells the plumber the layout they are working with, which speeds up finding the blockage or the break.
How this connects to the 2024 changes
The drainage plumbing diagram is part of the certification now required for property sewer connection work. Since 1 July 2024, drainage plumbing that includes the sewer connection has to be done under a licensed plumbing contractor and certified with the Plumbers Licensing Board, and the diagram is one of the documents in that process, alongside the notice of intention, completion notification and certificate of compliance. We cover that in full in our guide on sewer connections and drainage responsibility.
Sanitary drainage is governed by AS/NZS 3500.2 under the Plumbing Code of Australia, which is why this is licensed work with a documented compliance trail rather than something done and forgotten.
What it means for a buyer
On a purchase, it is worth knowing whether a drainage plumbing diagram exists, particularly if you have any plans to build or landscape. In older Perth suburbs, ageing clay drainage and large street trees make root intrusion and pipe deterioration common, and the diagram plus a plumber's drain-camera inspection together give you a picture of an area a visual building inspection cannot reach.
A pre-purchase inspection under AS 4349.1 reports the visible signs of a drainage problem, but concealed drainage is outside its scope, and where the evidence or the property's age warrants it, a licensed plumber's drain inspection is the right next step. The diagram tells that plumber where to look.
What the diagram records and why it exists
A drainage or plumbing diagram is the as-built record of where the drainage actually runs on a property: the layout of the pipes, the connection points, the fixtures they serve, and the route the waste takes to the sewer. It exists because drainage is buried and invisible, and without a record, anyone who needs to work on it, or who needs to build over or near it, is working blind. When a plumber is called to a blockage, when an owner wants to add a bathroom, or when someone plans an extension that might sit over a drain, the diagram is what tells them what is down there and where, so they are not excavating on guesswork.
For most owners the diagram is invisible until the moment it is needed, and then its absence is a genuine problem. Building over or too close to a drain without knowing it is there can mean either damaging the drain or, later, having no access to it when it fails. Renovating a wet area is far simpler when the existing drainage layout is known. Even selling can be affected, because a buyer's due diligence, or their own future plans, may turn on knowing how the property is drained. The diagram is one of those documents nobody thinks about until they urgently need it. Then it matters a lot.
How to obtain one, and when the effort is worth it
Drainage diagrams for a property can often be obtained, and where one exists it is worth having in the file, particularly before any work that involves the drainage or building near it. For a straightforward purchase of an established home with no renovation plans, the diagram is a nice-to-have rather than a necessity. Where it earns its keep is when you are planning work: an extension, a new wet area, anything that will sit over or connect into the existing drainage, or any situation where recurring drainage problems suggest you need to understand the layout to diagnose them. In those cases, having the diagram, or commissioning the investigation to establish the layout where no reliable diagram exists, saves both money and grief.
It connects, too, to the broader point about buried drainage risk. Where a property is old, on reactive soil, or heavily treed, and there is reason to be concerned about the condition of the drains rather than just their layout, a camera inspection of the drains does two jobs at once: it reveals the condition and it helps establish the actual run of the pipes. For a buyer weighing an older property with drainage question marks, that combination of understanding where the drains go and what state they are in is often the information that matters most, and it is entirely separate from what a visual building inspection can see.
Why buried services matter to a buyer
The recurring theme with drainage, whether it is the connection, the condition, or the diagram, is that it is buried, expensive, and outside what an ordinary inspection can assess, which makes it one of the genuine blind spots in a standard property purchase. The house can pass a building inspection with flying colours while the drainage beneath the garden is root-invaded and failing. They are not the same assessment. Understanding this is what stops a buyer from assuming that a clean building report means the whole property is sound. Where the age, the soil, the trees, or the symptoms give reason for concern, the drainage deserves its own specialist look, and the modest cost of establishing its layout and condition before settlement is small against the cost of discovering a collapsed or misrouted drain afterwards, when both the repair and the disruption fall to the new owner.
Have it before you need it
The recurring lesson with the drainage diagram is that its value is almost entirely in having it before the moment you need it. Nobody misses the diagram until they are planning an extension over an unknown drain, diagnosing a recurring blockage, or trying to work out why a wet-area renovation is more complicated than expected, and at that point its absence is a real and sometimes costly obstacle. For a straightforward purchase with no plans, it is a low priority. But if you are buying with renovation in mind, or taking on an older property where the drainage layout is genuinely unknown, establishing where the drains run, and while you are at it what condition they are in, is knowledge that pays for itself the first time you need to build near, connect into, or unblock the system. It is far easier to obtain or establish the layout calmly in advance than to work it out under pressure when something has gone wrong.
Common questions
What is a drainage plumbing diagram?
A record of the sanitary drainage layout on a property — where the drains run, the connections and inspection points, and where the system meets the main sewer. Building and Energy holds these as a reference for homeowners and plumbers.
Why do I need a drainage plumbing diagram?
Any time you disturb the ground — an extension, slab, pool, footings or major landscaping — the diagram shows where the drains run so you don't dig into a sewer line, which is a costly and messy repair to avoid.
How do I get the drainage diagram for my property?
These diagrams are held by Building and Energy and are used by homeowners and licensed plumbers to determine a property's drainage layout. They form part of the certification for sewer connection work.
This comes up on every pre-purchase building inspection and building stage inspections we carry out.
Read next: building permits in WA: BA1, BA2, and when you need one and sewer connection and drainage: where your responsibility runs to.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.