If you’re building a new home (or you’ve just bought a new house in a new development), stormwater is one of those topics you don’t think about until it goes wrong. Then it becomes very interesting, very quickly.
Rain falls on your roof and block, hits impervious surfaces and impermeable surfaces like roofs, driveways, paths, and patios, and becomes stormwater runoff (or storm water runoff—same thing, different spelling). In urban areas and dense residential areas, there’s typically less green spaces and more hardstand. That means more runoff, moving faster across the land surface, with less soaking in.
And here’s the part people routinely muddle: stormwater is not sewer.
Your sewer systems carry wastewater from showers, toilets, and kitchens. Stormwater should carry rain-related runoff away from the home. The paperwork and approvals are different, the risks are different, and a cross-connection can create expensive and unpleasant consequences. If you’ve been given sewer plans as part of your build pack, keep them—mainly because they help you understand where services run and where trades should (and shouldn’t) be connecting things.
Rainwater vs stormwater: what’s the difference?
Rainwater is what you choose to capture, usually from your roof, typically into a tank. People think of it as part of their water supply strategy—watering gardens, toilets, sometimes laundry. If you’re feeding internal fixtures, you’ll also hear terms like potable water and separation requirements, because the mains supply needs to be protected from back flow and contamination risks.
But rainwater storage isn’t automatically stormwater management. It only reduces runoff if the tank has spare capacity when the rain hits. If the tank is full, it overflows—and that overflow becomes stormwater, and it still needs a legal pathway into the stormwater system.
Stormwater is everything rain-related that runs off roofs and ground surfaces and needs to be safely managed. In many suburbs it ends up in pits and pipes that connect to council drainage. In other settings it may run to a lawful discharge point via civil drains.
The main goal is simple: stop water pooling around your slab and stop your block dumping uncontrolled water onto neighbouring property or public space.
What “on-lot detention” actually means
On-lot detention (often shortened to detention) means you temporarily store stormwater and release it slowly. The reason is straightforward: when an area gets developed, it gains more impervious surfaces. That causes sharper, faster surges of runoff. If everyone dumps their roof and driveway water into the street system at once, council drainage networks struggle.
So detention measures aim to flatten the peak flow. That might be done using stormwater detention tanks, or other underground detention systems and underground systems installed under the driveway or yard. In larger precincts you may see detention ponds and broader stormwater management ponds that manage flows at an estate level.
Detention isn’t about “soaking water away”
It’s about controlling the release rate. If the outlet control device is missing, bypassed, or blocked, the system either does nothing (it dumps too fast) or it becomes a bathtub (it fills and backs up). Either way, you’re the one living with it.
What about retention ponds, retention tanks, green roofs, and infiltration basins?
Retention is different from detention. Retention aims to keep water, not just delay it. At estate scale, retention ponds often hold a permanent pool and can help with water quality outcomes by letting sediment settle and supporting some treatment processes. At a household scale, you’ll sometimes see the term retention tank used to describe storage intended for reuse or longer-term holding, but the practical reality is still the same: everything needs a safe overflow path.
You may also hear about green roofs and expanded green spaces as ways to reduce runoff and slow flows. They can work, but they’re not common on typical detached houses unless the design brief is higher-end or planning-driven. More often, councils rely on detention requirements and estate-scale basins.
An infiltration basin is a soakage-style approach designed to let stormwater soak into the ground. Whether that’s suitable depends on the site, soil conditions, groundwater, slopes, and setbacks. If infiltration is used, it must be placed at a safe distance from footings and key services.
Poorly positioned infiltration can create persistent wet zones near the house, soften soils, and cause ongoing drainage headaches.
When do you actually need detention, retention, or infiltration?
Basic stormwater management is non-negotiable on a new home. Roof water needs collection and conveyance. Surface water needs sensible falls so it doesn’t pond against the slab. Even if you’re in rural areas, you still need a plan for where water goes and how it leaves the site.
Detention tends to appear when your local council has conditions that limit peak discharge from the lot. That’s common in urban areas and many new development estates, where cumulative runoff from many houses is a problem. It also shows up when downstream pipes are constrained or where flooding sensitivity is higher.
Retention or reuse strategies are more common where homeowners want better resilience or lower mains demand, but on many lots it’s still the council drainage network doing the heavy lifting once overflow occurs.
Infiltration depends heavily on site conditions and local regulations. Some sites simply can’t support it, and some councils restrict it.
If you’re unsure what your site requires, that’s a clue there’s a gap in communication. The requirement usually lives somewhere in approval conditions, engineering notes, or the drainage plan set.
What your plumber/builder should show you (the proof)
This is where most disputes could be avoided. It’s not enough to hear “stormwater’s done.” You want to see it and document it while it’s still visible.
- Ask for a two-minute walkthrough. Your supervisor or plumber should be able to point out the stormwater path from roof to discharge point. That includes downpipes, pipe routes, pits, and where it ultimately connects or discharges. If the explanation is vague, you should assume there’s risk.
- Ask for photos before backfill. Once trenches are covered, you’re relying on memory and goodwill. Get photos showing pipe routes, joints, changes of direction, pits, and any underground systems like detention storage. Make sure it’s clear where pits sit relative to finished levels. A simple tape-measure photo can be surprisingly helpful later.
- Make sure stormwater is not tied into sewer systems. It sounds basic, but mistakes happen. This is where keeping sewer plans on file helps you sanity-check service corridors and understand what’s meant to be where.
- If you have detention installed, ask to see the working components. A detention tank that’s just a tank with no functioning control is not detention. Your builder should be able to show you what device controls the outflow, where it is, and how it will be maintained. This is typically based on a design or specification that may come from a hydraulic engineer or council conditions.
- Ask where overflow goes. Every system needs an overflow strategy, whether it’s a rainwater tank, a detention setup, or an estate-scale basin. Overflow that spills toward the house is the kind of “small detail” that becomes a big problem.
Finally, ask about separation. Stormwater trenches and services often share tight corridors on residential sites. Confirm they’ve maintained a safe distance to potable water and the broader water supply lines wherever practical, and that infiltration-style measures (if used) aren’t saturating service trenches.
At handover, request a simple “drainage pack”: drawings, marked-up notes if anything changed on site, key photos, and a basic map of pit locations and main pipe runs. For a property owner, this is the difference between a quick fix later and an expensive investigation.
Common failures and what they look like later
Stormwater failures rarely announce themselves politely. They show up as flooding, ponding, erosion, or repeated blockages.
One classic issue is yard flooding even when the pit “looks fine.” That’s often a levels and falls problem across the land surface, or a pit set too high relative to surrounding areas. Water takes the path of least resistance, and if the low point is near the slab edge, that’s where it will sit.
Another common issue is garage or driveway ponding. On many builds, driveway levels and falls are tight. Add a poorly positioned pit or incorrect finished levels, and you get water pushing toward the garage. In estates, this can be worse because runoff moves quickly across hard surfaces.
Blockages are also common early on. Construction sediment, debris, and landscaping materials can clog pits and lines. Blocked catch basins reduce the system’s capacity and turn heavy rain into a ponding event. This is also where water quality issues show up—muddy discharge, silt in pits, and, more broadly, increased water pollution entering downstream waterways. That matters because councils are trying to protect natural habitats, and because blocked systems become your maintenance problem.
Detention failures are their own category. If the outlet control device is missing, bypassed, or blocked, the system either dumps too fast or backs up. Both outcomes defeat the purpose of detention and can cause unpleasant surprises.
And yes, cross-connections happen. If stormwater ends up connected into sewer systems, you can get odours and compliance issues. If sewer ends up into stormwater, it’s worse. Either way, it’s far cheaper to identify and prevent this than to fix it later.
Why builders and plumbers sometimes struggle
Stormwater is often installed early and then suffers death-by-a-thousand-cuts as the build progresses. Tight sites make pipe grades difficult. Driveway and landscape levels can change. Trades work in different sequences. Extra paving goes in later, increasing stormwater runoff beyond what the original plan expected.
None of this is an excuse for sloppy work. It’s the reason you insist on evidence before backfill, and why you keep records after handover.
Best practices that actually work
The best approach is boring and consistent: treat stormwater as a real system, get a walkthrough and photos before anything is buried, and keep stormwater separate from sewer. If detention is required, confirm the outlet control is installed, accessible, and protected from debris. If retention or infiltration is involved, confirm overflow and setbacks are sensible and that anything intended to infiltrate is kept a safe distance from footings and from water supply and potable water services.
Also, don’t ignore site cleanliness. Keeping sediment out of pits protects water quality, reduces blockages, and limits downstream water pollution. Even a single house build can send a surprising amount of silt into the street system if controls are poor.
Finally, think about the future. If you plan to add more hard surfaces later, remember you’re increasing storm water runoff. That can change how the system behaves and, in some cases, can conflict with assumptions in approvals and local regulations.
Conclusion
Rainwater, stormwater, and on-lot detention are connected, but they serve different purposes. Rainwater supports reuse and can reduce runoff when it has spare capacity. Stormwater must be safely managed off the block. Detention is often required in urban areas and new development estates to control peak flows from lots dominated by impervious surfaces.
The practical win is simple: don’t accept vague assurances. As the property owner, get a quick walkthrough, get pre-backfill photos of the new pipes and pits/catch basins, and keep the plans and any hydraulic engineer details in a drainage pack. You’ll save yourself time, money, and arguments when the first serious storm hits.
Frequently Asked Questions
1) What’s the difference between stormwater runoff and sewer?
Stormwater runoff is rain-related water from roofs and ground surfaces. Sewer systems carry wastewater from internal fixtures. They should stay separate.
2) Why do councils require detention on some blocks?
In many new development areas, the street network can’t take peak flows from lots with lots of impervious surfaces. Detention slows discharge to protect the system.
3) Are detention ponds and stormwater detention tanks doing the same job?
Broadly, yes—both manage peak flows. Detention ponds are usually estate-scale. Stormwater detention tanks and underground detention systems are often lot-scale.
4) What’s the point of retention ponds?
Retention ponds hold water longer, which can support improved water quality outcomes and reduce downstream impacts, including effects on natural habitats.
5) What is a retention tank on a house build?
A retention tank generally refers to storage intended to keep water for reuse or longer holding. But it still needs an overflow pathway into the stormwater network when full.
6) Is an infiltration basin always a good idea?
Not always. An infiltration basin depends on soil, groundwater, slope, and setbacks. Done wrong, it can create persistent wet zones near the house.
7) What should I ask for before trenches are backfilled?
Ask for a walkthrough, pre-backfill photos, and a basic sketch/map of pipe routes, pit locations, and any detention components—so you know what was installed and where.
8) Can nearby parking lots affect my stormwater?
They can. Parking lots add large impervious surfaces and can influence local flows and water quality, depending on the area and drainage layout.
9) Why do new pipes still end up blocked early?
Construction sediment, debris, and landscaping materials commonly clog pits and lines early on. A clean out before handover helps prevent this.
10) Who decides what system I need?
Your local council sets requirements via approvals and local regulations. If detention/retention is required, a hydraulic engineer may design or specify the system that gets installed.
Further Reading
Lissy Alvaran
Anthony Painter
Anthony Painter
Anthony Painter
Anthony Painter
Anthony Painter
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