Which Way Does Your Block Fall? Legal Point of Discharge, Detention, and Why Stormwater Sets Your Floor Levels in SA Infill

General information only for South Australian landowners. It does not constitute legal, planning, engineering, plumbing or valuation advice. Where your site may lawfully discharge stormwater, and on what conditions, should generally be confirmed with your local council. If the relevant drainage infrastructure is owned or controlled by another public authority or a private party, their approval may also be required. Always obtain the position in writing. Drainage design, flow calculations, detention and retention sizing and overland flow assessment belong to a civil or hydraulic engineer; levels, falls and boundaries to a licensed surveyor; easements and any drainage right over someone else's land to your solicitor or conveyancer; soil behaviour and water near footings to a geotechnical or structural engineer; and plumbing to a licensed plumber working to the current requirements. Council policies, design guidelines and the Planning and Design Code are amended from time to time, so confirm the live position with the relevant authority before you rely on anything here. Nothing in this article is a determination about your block.

Water only ever goes one way, and the direction was settled long before you bought the block.

Owners almost always arrive at this subject with a yes or no question. Do I need a detention tank? It sounds like it should have a short answer, and every person they ask gives them a different one. A neighbour did a one-into-two and needed nothing. A friend in another council area was made to put a tank under the driveway. The builder says it depends. The engineer says it depends on the council. The council says to submit a stormwater management plan.

None of them are being evasive. The question is the wrong shape. Stormwater on a small infill site is not a requirement handed down from a rulebook. It is the consequence of two facts about your particular piece of land — which way it falls, and what the drainage system at the bottom of that fall can accept — and everything else follows from those.

Which makes it an unusual constraint to carry, because there is nothing to go and find — nothing buried under the yard that a search reveals and a designer then draws around. What governs the scheme is not a place on the block at all. It is a height. Somewhere at or near your boundary sits a level at which your water is permitted to enter public infrastructure, and every level above it is worked backwards from that one: the last pit, the pipe grades, the yard, the driveway, the finished floor of each dwelling. You do not set that height, and nobody on your side of the fence does.

Two subjects get merged constantly here, and what separates them is a direction of travel. This article is not about the Flood Hazard Overlay. That is mapped hazard — water arriving on your land from somewhere else, and what a mapped overlay does to how your application is assessed, which we cover in what it means when your Adelaide block lands in a flood hazard overlay. This article runs the other way: water leaving your land, the level and the place at which it is allowed to go, and who owns the pipe it goes into. A block with no overlay on it at all can still have a serious stormwater problem.

Every one of those levels belongs to somebody, and none of them belong to us. Cyberate PM does not design drainage, calculate flows, size detention or retention, or nominate a floor level, and we do not speak for your council. We are engaged and paid by the owner — never on the transaction, never by a developer or a lender — so the only thing we are protecting on this subject is the order in which the questions get asked. Here that order is unusually unforgiving, because the height that governs everything else is held outside your project team entirely, and it is slow to obtain. We coordinate; we do not give the advice.

Inside the council, the drainage answer sits at a different desk

Owners reasonably assume that if there is a stormwater rule, it lives in the Planning and Design Code, alongside the zone and the minimum site area. That is not quite how it works, and the mismatch is what makes the subject feel arbitrary.

Planning policy is concerned with outcomes: that a development manages the runoff it generates, that it does not create a drainage problem for neighbours or downstream properties, and that water leaving a site is not carrying sediment and pollutants into the network or a watercourse. State policy work on residential infill has also pushed on-site water measures on new dwellings — the tank, roof-connection and soft landscaping side of the subject, which belongs with the broader infill reforms in how SA's tightened residential infill rules change your yield rather than here.

What planning policy does not do is tell you where your water may physically go. That is not a planning question at all. The local drainage network — the kerb, the side entry pits, the underground pipes, the open channels and the basins — is owned and operated by your council, with the state-level Stormwater Management Authority sitting behind local government as a planning, prioritisation and funding body rather than as the operator of your street drain. So the question may my site discharge into your system, and where is put to the council as the owner of that infrastructure, and the answer turns on what is in the ground near your frontage and what capacity it has left.

Two different questions, two different bodies of policy, frequently answered by two different people inside the same council. The confusion is understandable, because from outside the building there is one counterparty with one logo on the letterhead. From inside there are two: an assessment function that will judge your application on planning merit, and an asset owner that will say what its pipes can take. The drainage question usually gets put to the planning officer, who is not the person holding that answer, and a polite non-answer from the wrong desk is easy to mistake for a soft yes.

The point of discharge is a level before it is a place

Search this subject and you will land almost immediately on the phrase legal point of discharge, usually on a Victorian council's website, sometimes with an application form attached. That is Victorian usage, and over there it means whatever Victorian law and Victorian council practice make it mean. It sits in this article's title for one reason — it is the phrase owners actually type into a search box — and it is not a South Australian statutory term. Nothing here uses it as one. South Australian councils more often talk about the point of discharge, or connection to the council's system. What carries across the border is the practical question rather than the legal machinery behind the words: on a South Australian site you still need the location the council nominates and accepts as the place where drainage from your land enters public infrastructure. Whether the South Australian version of that carries any standing comparable to the Victorian term, and what your council calls it in its own material, is the council's to state and not something to read across from an interstate website.

Whatever the label, owners hear the wrong half of the phrase. They hear where, as though the job were to find a spot on a plan. The operative half is how low. The point carries a level, and a gravity system is only a chain of levels stepping down to it — each pipe falling towards the next, each pit sitting above the one below, the whole chain arriving at the council's level rather than somewhere convenient near it. Fix the bottom of that chain and you have fixed the top of it too, which is why a stormwater answer is felt in the height of a floor rather than in the position of a pipe.

It also fixes where the boundary of your responsibility sits, because drainage from the property to the road is generally the owner's to build and maintain, while the public network beyond it is the council's. That division is stated plainly in published council material, and it is the reason a shared or inter-allotment drain running through several back yards produces such durable arguments about who pays when it fails.

So on a sloping infill site the practical first step is usually not a tank decision. It is a written confirmation from your council of where the site may discharge, at what level, on what basis, and whether the receiving system can take more. That confirmation is a datum, not a formality — it is the number your surveyor and your engineer both need before either of them can tell you anything useful, and until it exists a concept plan is drawn on an assumption. Ask for it early, and treat a verbal indication over the counter as a lead rather than an answer.

Which way does your block fall?

Now the geography does the work.

If your land falls to the street and there is a kerb and an underground system in front of it, you are in the simple case. Water gets to the frontage under gravity, the connection is conventional, and the remaining questions are about how much and how fast — which is the next section.

If your land falls to the rear, or to a side boundary, or to a low corner with no public drainage anywhere near it, you are in the case that redesigns schemes. This is ordinary ground in Adelaide. The foothills suburbs behind Mitcham, Burnside and Tea Tree Gully throw fall in whichever direction the contour runs, not conveniently towards the road; across the flatter western and north-western plains the fall can be so slight that finding a workable gravity path becomes the governing constraint. Water does not run uphill to oblige a lot layout. The routes owners are commonly pointed towards from there are the ones below — a working list rather than a closed one, since a site can present a combination, a variant or something none of them describes — and each costs something other than money.

You may be able to drain to the rear, if a public drain exists behind the allotment. Rear-of-allotment drainage is real infrastructure in parts of Adelaide, but where a public drain runs through private land it generally sits in an easement in the council's favour, so connecting to it, extending it or building near it is a conversation with the council and a matter for your surveyor and solicitor to resolve on the plan and on the title.

You may be able to obtain a private drainage easement over a downstream neighbour's land — a negotiation with a third party who has no obligation to agree, on terms your solicitor drafts, and not something to assume inside a feasibility.

You may be able to hold the water on site and release it slowly, or hold it and not release it at all. That is detention and retention, and it is a common landing point on infill sites.

You may be able to let it soak away. Soakage depends on what the ground is, and much of Adelaide's inner and middle suburbs sits on reactive clay that does not accept water readily and that moves as its moisture content changes — which is why infiltration near a building is a geotechnical question rather than a drainage one, and belongs with the people who produce your site classification and footing design rather than with a drainage contractor.

Or you may be looking at pumping, which is the option to be most careful about — and this is the point to introduce the worked example this article leans on more than once. There is no single South Australian rulebook to quote from on any of this, so what follows uses one metropolitan council's published guidelines as an illustration of how a council reasons about the problem: the City of Playford's stormwater guidelines (v1.1, April 2025), chosen because that document is public and readable rather than because it speaks for the state or for your council. On pumping, those guidelines take the position that where the option of a gravity drain exists, a pumped solution will not be considered. The reasoning is easy to follow: a pump needs power and maintenance and eventually fails, and the council would be wearing the consequences downstream. Whether your own council reasons its way to the same place is its to say — but do not assume a pump can rescue a layout that gravity will not support until it has said so.

The comparison your engineer is designing to

Whatever route the water takes, one principle runs underneath the whole subject, and it explains almost every requirement an engineer will hand you.

Development increases the amount of water leaving a site and the speed at which it arrives, because roof, driveway and paving replace lawn and garden that used to absorb and slow the rain. The street system in an established Adelaide suburb was sized for the catchment as it was, not as it is becoming, and the compounding effect of many small infill sites in one catchment is exactly what worries a council engineer.

So the test the guidance works to is comparative rather than absolute. The worked example introduced above puts it in those terms: the post-development design should mimic the natural runoff regime, matched to the pre-development condition in volume as well as flow rate, with storage sized so the developed peak does not exceed what the site produced before — or, where the council has set a permitted discharge rate for the location, designed to that rate instead. That is one council's formulation, in one published document. Other metropolitan councils publish their own guidelines and nominate their own design storms, so which comparison applies to your site, and the event it is measured against, is your council's to state and your engineer's to design to. Detention systems exist to make that arithmetic work — you catch the extra water, hold it, and let it out at the slower rate.

You will not find the sizing here, and a general article is not the place to look for it. The design event, the permitted discharge rate, the storage volume and the treatment requirements are site-specific and council-specific, calculated by your engineer against the council's current guideline. A figure lifted from another council's document, or another state's, is not a safe starting point for yours.

Detention and retention are a design conversation, not a product you buy

Two words get used interchangeably and should not be. Detention is hold and release: the water is stored temporarily and discharged slowly, so the peak is flattened. Retention is keep: the water is captured and used, or infiltrated, and does not return to the network at all. They solve different problems — detention protects the downstream system from a peak, retention reduces the total volume and can supply the property — and a good deal of South Australian policy work has been about getting both out of the same tank.

Which is where the owner's instinct misreads what is happening. The instinct is to treat the tank as an item — something priced, ordered and dropped in near the end, like a hot water service. It is not an item. It is one term in an equation your civil engineer is solving, and the other terms are things you care about a great deal: how much roof and paving the scheme creates, where storage can physically sit, how it empties down between events, and whether the water is being got rid of or kept and used.

That makes it a conversation rather than a verdict handed down to you, and the conversation has a shelf life. An engineer brought in after the plan is fixed can only solve for the volume with whatever room the plan left over. An engineer brought in while the layout is still moving can tell your designer where storage would sit most cheaply, what the scheme would give up to put it there, and whether trimming paved area is a better trade than enlarging a tank. That is a different quality of answer, and it is only available before the drawing hardens. What comes out of it is the engineer's conclusion on your site's own numbers — not the designer's, not the builder's, and certainly not ours.

It comes out in the floor levels

This is the part where the subject stops being about drainage at all.

Levels lose their flexibility first, and they lose it all at once. Once the discharge point is fixed, the finished floor level of each dwelling has to sit high enough to drain to it, with the site graded to suit. That one constraint propagates faster than owners expect: raise a floor and you have changed the entrance, the steps, the driveway grade, the retaining and the relationship to the neighbours' land — which is how a stormwater constraint turns into a retaining wall conversation, with its own liability and cost consequences that we set out in who actually pays for retaining walls on sloping Adelaide blocks. A dwelling whose floor has been lifted is a different building: different height above the street, different outlook, different overlooking of a neighbour, different steps at the door. None of that was a design decision. It was the consequence of a level somebody else nominated.

Overland flow needs a route as well: every system has a capacity, and the water that exceeds it must travel somewhere on the surface without inundating anyone, which councils generally want running through public land rather than across private allotments.

Storage then has to physically exist somewhere. A detention tank goes under the driveway or in a yard, competing with the parking, the private open space, the tree policy now wants planted, and the setbacks. On a narrow infill allotment there is not much slack, and the tank is often the thing that finally settles where the driveway runs.

Put those together and drainage becomes a feasibility risk in its own right, not only a design detail. Where the rear allotment in a proposed one-into-two has no apparent gravity path to a lawful discharge point, no easement anyone is obliged to grant and ground that may not accept water, the layout carries a real risk of not being achievable — however comfortably the site area and frontage clear the tests in the zone. Whether it can in fact be made to work is not a question the zone tables answer, and it is not ours to answer either: the council confirms where and whether the site may discharge, a civil engineer tests whether a compliant drainage design exists for that layout, and a surveyor establishes whether the levels support one.

In our experience it is also one of the constraints that most often sends a small Adelaide infill scheme back to the drawing board after planning consent has already been granted — not because the planning system changed its mind, but because the water had nowhere to go, and finding it somewhere moved the driveway, moved the floor levels and sometimes moved a lot off the plan. That is a practice observation rather than a measured frequency, and other projects will have other stories.

It also does not end at consent. Where a division requires drainage infrastructure, easements created on the plan, or works connecting to council infrastructure, the council's satisfaction with those works becomes part of what has to be cleared before new certificates of title issue — the stage we describe in why an approved subdivision still has no titles. A stormwater problem found late is not just a redesign; it is a redesign dropped into the part of the program where the holding costs are already running.

Why the answer differs between councils — and why one council's document cannot answer for yours

An owner who talks to three people gets three answers partly because the councils genuinely differ. Each publishes its own engineering guidelines, sets its own design events and discharge limits, and holds its own view about what its downstream network can absorb — and state-level policy work has aimed at standardising some of the on-site water measures precisely because the variation is well known. There is usually a reason behind the difference: a council whose system is at capacity, or which discharges into a sensitive receiving environment, has grounds to ask more than a council with headroom.

Which is also the honest caveat on this article. The guidelines quoted above belong to a single metropolitan council. They are used here as a worked example — a readable illustration of how a council frames the problem — and not as a statement of the South Australian position, because on these questions there is no single state position of that kind to state. Your own council may frame the comparison differently, nominate a different design event, take a different view of pumping, or expect a different depth of information from a small division. Nothing in this article should be carried across a council boundary without checking.

By the same token, another council's interpretation, or what a neighbour was allowed to do, is a poor guide to your own site. Ask your council, in writing, before you design: where and at what level may this site discharge, what design event and discharge rate apply, what must the stormwater management plan show for a project this size, and is the receiving system constrained. The same published guidelines make that invitation in plain terms: for smaller divisions the level of information required may be lower, and prior to lodgement the council can advise what it will actually need. Whether your council extends the same invitation is worth finding out, because it costs nothing to take up and it is easy to miss.

How Cyberate PM handles this on your project

Read back over the sections above and almost every answer in them belongs to somebody who is neither you nor us: the council's drainage side, a civil engineer, a licensed surveyor, a geotechnical engineer, a solicitor, a licensed plumber. We are none of those. We do not design drainage, calculate flows, size detention or retention, prepare a stormwater management plan, set a floor level or certify anything, and we do not speak for your council. We hold no view on what your site will be required to do, and a view from us would not be worth anything.

What we hold is the chain — and on this subject the chain has a direction, because it is assembled from the bottom up. Everything is measured back from the lowest point, so that is where we start.

The survey goes first, and it goes briefed: fall across the whole site, existing drainage, neighbouring levels, enough of the surrounding ground to show where the water has been going all along. The discharge enquiry follows, addressed to the council's drainage side rather than to the planning officer who will one day assess the application, because those are two different desks and only one of them holds this answer. We put the question in writing and we ask for the answer in writing, with a level in it.

Only then does a designer have something firm to draw against. We brief the designer and the civil engineer off the same survey and the same council response, at the same time, so storage, driveway and floor levels get argued out between them — the conversation described earlier — instead of each being handed down to the next person as a settled constraint. Where an easement over someone else's land turns out to be one of the routes, we bring the solicitor and the surveyor in while it is still one option among several rather than a rescue attempt. And because the drainage works and the council's clearance of them sit past consent, they go onto the program as their own line items, with an owner and a date, instead of surfacing in the month you expected titles.

None of that makes the council's answer more generous. It makes the answer arrive while the drawing is still cheap to change, and it keeps two different council conversations from being mistaken for one.

Frequently asked questions

Do I need a detention tank to subdivide in Adelaide? There is no general answer that can be given without knowing your block. Whether storage is required depends on where your site can lawfully discharge, what the receiving system can accept, how much additional runoff your scheme creates, and your council's current requirements. Confirm the discharge point and requirements with your council in writing, then have a civil engineer assess the site — in that order, and before you commit to a layout.

What is a "legal point of discharge", and does South Australia use that term? The phrase is most common on Victorian council websites, which is why it dominates search results. It is not a South Australian statutory term, and this article does not use it as one — it appears here because it is what owners search for. South Australian material more often refers to the point of discharge, or connection to the council's system. The practical question behind both is the same one: you still need the location the council nominates and accepts for drainage leaving your land to enter public infrastructure. Whether the South Australian version carries any comparable legal standing is a separate question, and not one to assume from Victorian material. Whatever it is called locally, ask your council to confirm it in writing rather than inferring it from where your downpipes currently run.

My block falls away from the street. Can I still get a second allotment out of it? Sometimes, depending on whether a lawful path for the water exists. The routes usually considered include a public drain at the rear if one is there, a private easement over downstream land if a neighbour agrees and your solicitor documents it, storage on site, soakage where the soil genuinely permits it, or in limited cases a pumped system if the council will accept one — and an engineer looking at your particular site may see a combination of those, or something else again. If no lawful path can be established, the rear allotment may not turn out to be feasible even though the site area and frontage are — but that is a conclusion for the council and your civil engineer to reach on the evidence, not one to draw from the zone tables. Have the discharge question put to the council in writing and the site looked at by a civil engineer and a surveyor before you buy, price or design.

What is the difference between detention and retention? Detention holds water temporarily and releases it slowly, so the peak leaving your site is held to whatever rate the council's guideline allows — commonly the rate the site produced before it was developed. Retention keeps the water — for reuse, or by infiltration — so it never returns to the network. They address different problems and they are sized differently, so a system provided for one purpose will not automatically answer a requirement for the other — though some tanks are configured to do both. Which one your council requires is a question for the council, confirmed in writing; your civil engineer then designs to it.

Can I just pump the stormwater out to the street? Do not assume so. The worked example this article uses throughout — the City of Playford's stormwater guidelines, as published at the time of writing — takes the position that where the option of a gravity drain exists, a pumped solution will not be considered, because pumps need power and maintenance and eventually fail, and the consequences land downstream. That is one metropolitan council's stated position, not a South Australian rule, and other councils may or may not reason the same way. Which is exactly why it is a question for your own council. Pumping is a possibility to raise with them, not a fallback to design around.

When should I be asking about this? Before you pay for a concept plan, and ideally while your contract to buy is still conditional — how that is framed, and whether a condition can accommodate it, is a question for your conveyancer or solicitor. The discharge enquiry costs you a letter and some patience. Discovering the answer after planning consent can mean paying design fees twice, and often costs something in the layout as well — a driveway position, a floor level, or in the worst case a lot.

If your block falls the wrong way and you would rather know the level your design has to work back from before the design money is committed, we can sequence the council enquiry, brief the civil engineer and surveyor off the same information, and keep the works and clearances tracked through to titles. We coordinate your professionals and keep your project organised; we do not design your drainage or decide what your site will be required to do. Book a free consult.