Water on site: drainage, soakwells, low spots and roof runoff
Find out where every litre goes before you change a single level. Roof areas, downpipes, existing soakwells, surface falls, the puddle that never dries, and whether your neighbour's stormwater is your problem.
Every landscape renovation moves water. You pave the side access and 40 square metres that used to soak away now runs somewhere. You lift the alfresco slab 100 mm to meet the door threshold and the fall reverses. You take out a garden bed and the sand that was quietly absorbing a downpipe overflow becomes a compacted path.
None of that is visible on a dry day in February. All of it shows up in the first week of June, usually as a complaint from next door.
This lesson establishes what your block does with water right now, before you change anything. You are inheriting somebody else’s drainage decisions, and about half of them will be wrong.
The rule that governs everything
In Perth, stormwater is your problem and it has to stay on your land.
The national performance requirement is NCC 2025 clause H2P1: surface water from a 5% AEP storm that is collected or concentrated by a building or sitework must be disposed of in a way that avoids the likelihood of damage or nuisance to any other property. “Sitework” includes your paving, your retaining wall and your regraded lawn.
Councils then say it more bluntly. The City of Joondalup states that property owners have a statutory obligation under common law precedents and the Local Government Act 1995 to confine water runoff within their property boundaries. The City of Rockingham requires all stormwater to be disposed of within the confines of the subject lot without detriment to any adjoining land.
Some councils will let you connect an overflow to their drainage network — the City of Canning permits detention on site discharging to a lot connection pit at a maximum of 4 litres per second. Most will not, and none of them will let you push it over the fence. Confirm your own council’s position before you design.
What a professional does at this stage
A landscaper who has been sued once does four things before drawing anything.
Walks the site in the rain. Not after — during. Twenty minutes in a June downpour tells you more than a week of measuring. If you cannot wait for rain, run a hose at full bore into each downpipe for ten minutes and watch.
Probes for the soakwells rather than assuming. Existing capacity is money. Finding two serviceable 1200 mm wells means you may only need to add one, not four.
Measures impervious area off the plan, not by eye. Everyone underestimates roof area by 20 to 30 per cent because they forget eaves, carports and the patio roof.
Writes down what already fails. The corner that stays wet, the downpipe discharging onto the paving, the neighbour’s driveway falling toward your side gate. These become constraints on the plan, and later, line items in the quote.
This happens now rather than later for one reason: retrofitting drainage under finished paving is the most expensive rework in landscaping.
Step 1 — find and assess the existing soakwells
Start at the downpipes. Every downpipe either enters the ground (there is a soakwell, somewhere) or discharges onto the surface (there is not).
Follow the likely line out from the base of the downpipe. Soakwell lids are usually 50 to 200 mm below the surface and get buried under mulch, turf or a later paving job. Push a 1.2 m steel rod into the ground on a grid at roughly 300 mm centres until you hit a lid. In summer, look for a circular patch of turf that dries out differently from the rest, or a shallow slump where the backfill has settled.
Once you have the lid off, record:
- Diameter and depth — this gives you capacity from the table below
- Silt and root level — a well half full of sand and jacaranda roots has half its capacity, and most Perth soakwells have never been cleaned out
- Water level — standing water in a soakwell in October means the base is at or below the watertable
- Which downpipes feed it, and whether wells are linked
Older Perth houses frequently have no soakwells at all. That is survivable in deep sand, but it means your entire roof runoff is currently infiltrating at the drip line — and a new paved courtyard in that spot removes the infiltration area without replacing it.
Step 2 — calculate your impervious area
Impervious area is roof plus anything water cannot soak through: driveway, paving, concrete alfresco, pool surround and pool water surface.
For roof, use the plan area — the footprint including eaves — not the sloped surface. Take it off the building plans if you have them, or measure the footprint on site.
Then apply your council’s coefficient. Two verified Perth examples:
| Council | Design storm | Formula |
|---|---|---|
| Town of East Fremantle | 20 year ARI overflow | impervious m² × 0.0125 = m³ storage |
| City of Canning | 1 in 20 year ARI (with overland flow path) | impervious m² × 0.0150 = m³ storage |
The 20 per cent spread between those two is why you check yours rather than using a number off a supplier’s website.
Worked example. A Bayswater house with a 180 m² roof and 60 m² of paving has 240 m² impervious. At 0.0125 that is 3.0 m³ of storage required. Existing on site: two 900 × 900 wells, 0.57 m³ each, so 1.14 m³. Shortfall 1.86 m³ — about two more 1200 × 1200 wells, before you add a single square metre of new paving.
| Soakwell (dia × depth, mm) | Capacity (m³) | Impervious area served (m²) |
|---|---|---|
| 900 × 600 | 0.38 | 30 |
| 900 × 900 | 0.57 | 46 |
| 1200 × 600 | 0.68 | 54 |
| 1200 × 900 | 1.02 | 82 |
| 1200 × 1200 | 1.36 | 109 |
| 1500 × 1200 | 2.10 | 168 |
| 1800 × 1200 | 3.05 | 244 |
Figures from the Town of East Fremantle information sheet, at 0.0125 m³ per m².
Step 3 — map the surface water
Take your measured plan from measuring up and mark, in a different colour:
- Arrows showing the direction of surface fall in each area
- Every place water ponds, with a rough depth and how long it takes to clear
- Every point where water crosses a boundary, in either direction
- Downpipes discharging to the surface
- Any low point that has no way out
Photograph ponding within 30 minutes of the rain stopping, then again 24 hours later. The difference between those two photos is your infiltration rate, and it is more useful than any calculation.
Why Perth soakwells fail
This is where local knowledge separates from generic drainage advice.
The winter watertable. The Department of Water’s own soakwell guidance says the base must be above the maximum or controlled groundwater level. On the Bassendean flats — Bayswater, Bassendean, parts of Gosnells, Forrestfield, inland Wanneroo — the watertable can sit within a metre of the surface in September. A soakwell standing in groundwater has no infiltration capacity at all, which is why the same well that works in March overflows in July. Check your address on the Perth groundwater map before you decide anything.
Perched water on coffee rock or clay. If your test hole from the Perth soils lesson hit a cemented coffee rock layer or clay, water will sit on it. Deep sand rules do not apply.
Silt and roots. No sediment control at the downpipe, no grated inlets, lids buried under turf so nobody has ever opened them — then twenty years of leaf litter and washed-in sand.
Compacted builder’s fill. The strip beside the driveway is usually the worst-draining ground on a Perth block, and it is where the soakwells are.
The same guidance also requires a design that leaves no water ponding after 96 hours between November and May, because of mosquito breeding. A puddle that lingers for a week in summer is not just ugly.
What this changes downstream
Four numbers come out of this lesson and get used repeatedly:
- Existing soakwell capacity (m³) → how much new drainage you actually buy
- Impervious area now, and after your design → the sizing calculation
- Ponding locations and clearance times → your grading plan and planting
- Boundary discharge points → a constraint you cannot design around
Carry the ponding and fall observations into levels, falls and datum so the survey confirms what you saw in the rain, and into the constraints and opportunities plan. The design work happens in the drainage plan, and the pits go in the ground in drainage and soakwells.
The next lesson finds the other things buried out there — locating services — which matters immediately, because the trench you dig for a new soakwell line is the one most likely to hit a gas service.
Checklist
- Locate and probe every existing soakwell
- Calculate your total roof and paved (impervious) area
- Map where surface water flows and ponds after heavy rain
- Check no runoff discharges onto a neighbouring property
Where to go deeper
Every link below was fetched and checked before it shipped. Nothing here is a guess.
- Freegovernment10 minPerth / WA
Information sheet — stormwater drainage general requirements
City of Canning
Why this oneThe clearest published statement of how a Perth council actually sizes on-site drainage. It gives you the design storm (1 in 20 year ARI where an overland flow path exists, 1 in 100 year where it does not), the coefficient to multiply your impervious area by (0.0150), the rule that lots under 300 square metres must use the whole lot area, and a full soakwell diameter-depth-capacity table. It also defines an overland flow path as 300 mm of clearance between road level and lot level.
Change this for PerthThis is Canning's specification, not a state-wide rule. Councils differ by roughly 20 percent on the coefficient alone. Use it to understand the method, then get your own council's number before you order anything.
Link checked 2 Aug 2026
- Freegovernment10 minPerth / WA
Requirements for onsite stormwater drainage (v3.0)
Town of East Fremantle
Why this oneA two-page sheet that hands you the whole calculation: impervious area in square metres multiplied by 0.0125 equals the storage capacity required in cubic metres, plus a table converting every common concrete soakwell size straight to the impervious area it serves. It also states the minimum soakwell allowable for roof water (900 mm diameter by 600 mm deep), the setback rule (no closer than 1 m or its own depth, whichever is greater, from a footing or boundary), and that overflow outlets are required for the 20 year ARI storm.
Change this for PerthEast Fremantle sits on Spearwood sand and limestone. The formula is generic across Perth councils, but the soakwell setback and linkage rules are that Town's own policy — confirm the equivalents with your council.
Link checked 2 Aug 2026
- Freegovernment8 minPerth / WA
Residential soakwells (stormwater runoff)
City of Joondalup
Why this oneThe plainest statement of the legal position you will find on a council website — property owners have a statutory obligation under common law precedents and the Local Government Act 1995 to confine water runoff within their property boundaries. It also gives practical numbers homeowners never think about: roughly one downpipe per 15 square metres of roof, downpipes no more than 12 m apart, grated inlets at the base of every downpipe, and trafficable lids and bases on any soakwell under a driveway.
Change this for PerthJoondalup is deep Bassendean and Spearwood sand where infiltration is easy. If you are on clay in the foothills or a wet winter watertable on the flats, the sizing guide will be optimistic.
Link checked 2 Aug 2026
- Freestandard10 minAustralian
NCC 2025 Volume Two, Part H2 — Damp and weatherproofing (H2P1)
Australian Building Codes Board
Why this oneThe national performance requirement sitting underneath every council soakwell policy, readable free online. H2P1 requires that surface water from a 5 percent AEP storm collected or concentrated by a building or sitework be disposed of in a way that avoids the likelihood of damage or nuisance to any other property, and that surface water from a 1 percent AEP storm must not enter the building. That single sentence is why you cannot grade your new courtyard toward the fence.
Change this for PerthWA adopted NCC 2025 on 1 May 2026, with NCC 2022 still usable for building permit applications during the transition. The H2P1 wording is unchanged between the two editions.
Link checked 2 Aug 2026
- Freearticle5 minPerth / WA
Soakwells — water sensitive urban design fact sheet
New WAter Ways / Department of Water
Why this oneFour pages from the state water department on why soakwells work here and when they stop working. The two lines that matter most on a Perth block are that the base must be above the maximum or controlled groundwater level, and that the design must not leave water ponding after 96 hours between November and May because of mosquito breeding. It also flags sediment control as the main cause of blockage — which is exactly what has silted up the soakwells you are about to probe.
Change this for PerthWritten for subdivision and street-scale water sensitive urban design as much as for single lots, and the branding predates the merge into DWER. The design factors apply unchanged to a suburban block.
Link checked 2 Aug 2026
- Freetool5 min for your addressPerth / WA
Perth groundwater map
Department of Water and Environmental Regulation
Why this oneFree, official, and the fastest way to find out whether your soakwells will be sitting in groundwater in September. Type your address and it estimates depth to the watertable, depth to the base of the superficial aquifer and likely salinity. On the Bassendean flats this single number explains most of the drainage failures people blame on their builder.
Change this for PerthIt is a regional model, not a bore log for your block. It gives you an estimate and a warning, not a design figure — for a difficult site you still want a geotechnical report.
Link checked 2 Aug 2026
- Freegovernmentseveral hoursPerth / WA
Stormwater management manual for Western Australia
Department of Water and Environmental Regulation
Why this oneThe deep end, free, in eleven downloadable chapters. Chapter 6 covers retrofitting water sensitive urban design into existing developed areas, which is exactly the situation on a renovation block, and Chapter 9 covers structural controls including biofiltration and infiltration systems. Read it when you want to understand why the coefficient in your council's fact sheet is the number it is.
Change this for PerthWritten for local government engineers and subdivision designers. Chapters 3 and 4 are superseded by the 2017 Decision process for stormwater management in Western Australia. Skim for principles rather than reading cover to cover.
Link checked 2 Aug 2026
The rules, with sources
WA regulation changes. Each point below names its primary source and the date it was last checked — always confirm with your own council before you build.
Property owners have a statutory obligation under common law precedents and the Local Government Act 1995 to confine water runoff within their property boundaries, and all stormwater falling within the lot boundaries must be contained on site.
City of Joondalup — Residential soakwells (stormwater runoff) ↗ · checked 2 Aug 2026
All stormwater is required to be disposed of within the confines of the subject lot without detriment to any adjoining land.
City of Rockingham — Soakwells ↗ · checked 2 Aug 2026
NCC 2025 Volume Two clause H2P1 requires that surface water from a storm with a 5 percent annual exceedance probability, collected or concentrated by a building or sitework, be disposed of in a way that avoids the likelihood of damage or nuisance to any other property, and that surface water from a 1 percent AEP storm must not enter the building.
Australian Building Codes Board — NCC 2025 Volume Two Part H2 ↗ · checked 2 Aug 2026
City of Canning sizes on-site drainage for a 1 in 20 year ARI storm where an overland flow path is provided, using a drainage coefficient of 0.0150 multiplied by the impervious area of the lot; where no overland flow path is provided the development must manage a 1 in 100 year ARI storm and a certified stormwater management plan by a qualified engineer is required. Lots under 300 square metres must use the entire lot area in the calculation.
City of Canning — Information sheet, stormwater drainage general requirements ↗ · checked 2 Aug 2026
Town of East Fremantle requires soakwell capacity calculated as impervious area in square metres multiplied by 0.0125 to give capacity required in cubic metres, sets the minimum soakwell for roof water at 900 mm diameter by 600 mm deep, and requires soakwells to be no closer than 1 m or their own depth, whichever is greater, from a footing or boundary line.
Town of East Fremantle — Requirements for onsite stormwater drainage v3.0 ↗ · checked 2 Aug 2026