Detention Or Dead Space? What Stormwater Really Costs A Small Site

STORMWATER STORAGE CAN TAKE YOUR BEST SITE SPACE.Price detention, layout and drainage before trusting the yield.

The site is level, the zoning works, the duplex numbers stack up. Then you walk it after a wet week and there is standing water in the back corner. Your drainage consultant mentions on-site detention. You nod, and then you realise you do not know whether that costs you five thousand dollars or one of your bedrooms.

Stormwater is the most underestimated constraint on small infill development in New South Wales, because it is both a cost and a use of land. On a tight site, the second one is what kills the deal.

The deal question: how much of my site does the water take?

Most developers treat stormwater as a plumbing matter handled at construction certificate stage. It is not. On-site stormwater detention is a planning requirement in many catchments, it must be designed and certified before you lodge, and the storage has to physically fit somewhere on the land.

The right due diligence question is not “do I need detention?” It is: how much storage volume, how much land area to hold it at the permitted depth, and what does that do to my yield?

The checks a student would run, in order

1. Find out whether the lot is in a detention catchment at all. This is not a general rule; it is mapped. Penrith City Council's stormwater specification (policy ES 002) lists the specific local catchments where on-site detention is mandatory, including Penrith CBD, Penrith North, Lemongrove, Emu Plains, Jamisontown, St Marys, Kingswood and Orchard Hills. If your site is outside those catchments, the obligation changes.
2. Check the flood and overland flow position. Overland flow is separate from detention and often more restrictive. Penrith's specification requires minimum freeboard of 500 millimetres above the 1% AEP flood level for residential floor levels. Bayside Council's Part 10 stormwater guidelines require a flood study or overland flowpath assessment with the development application where the site adjoins an identified major overland flow path, sits at or near a catchment sag point, or has any part below 4 metres AHD. Bayside also provides that no structures or fill are permitted over the 1% AEP overland flow path without approved mitigation.
3. Get the storage number, not a guess. Where a simplified method applies, some councils publish the parameters. Penrith publishes permissible site discharge and site storage requirement rates by land use — for multi-unit housing, 120 litres per second per hectare discharge and 240 cubic metres per hectare of storage. On an 800 square metre site, that is roughly 19 cubic metres of storage. Sydney Water's on-site stormwater detention guidelines note that Sydney Water will calculate and advise the permissible site discharge and site storage requirement for a given address when supplied with the site area and the pre- and post-development impervious areas.
4. Check how shallow the storage has to be. This is the step that decides the land take. Penrith caps above-ground storage depth at 200 millimetres in hardstand areas used for parking and pedestrian access, and 600 millimetres in landscaped areas. Nineteen cubic metres at 200 millimetres needs roughly 95 square metres of surface. Even at 600 millimetres, it needs over 30 square metres. That is where a car space or a courtyard goes.
5. Find out where storage is not allowed. Penrith states that storage is not permitted in private courtyards, overland flow paths or floodways, natural watercourses or effluent disposal areas. A bypass allowance exists but caps how much of the site may drain elsewhere — 15% of total site area under both Penrith's and Bayside's guidelines.
6. Confirm the discharge point and its capacity. City of Sydney's stormwater drainage manual limits the maximum permissible site discharge to 25 litres per second for storms up to the 20-year ARI. Penrith applies a maximum of 25 litres per second at any single discharge point. If the kerb cannot take the flow, you may be extending or upgrading the council's network at your cost.
7. Check the title consequences. Detention is not just a cost; it can encumber the title. City of Sydney's manual states that “a positive covenant must be registered on the title of the land for all drainage systems involving OSD to ensure maintenance of the approved OSD system, regardless of the method of connection”. Bayside requires the stormwater management plan and design certification to be prepared by an accredited professional as part of the development application.

The order, and why it is the whole lesson

Do it this way and each step narrows the next. Catchment first, because it tells you whether you have an obligation at all. Flood and overland flow second, because they can override everything else — a flow path through the middle of your site changes the usable area before detention even gets considered. Numbers third, because they let you calculate land take. Depth fourth, because depth is what converts a volume into an area. Title last, because it is the thing that outlives the project.

Reverse this order — design the building, then ask a civil engineer to make the stormwater fit — and you will be redrawing the scheme at documentation stage, which is the most expensive possible time to redraw it.

The trap: measuring the cost in dollars instead of square metres

The emotional trap on stormwater is fixating on the invoice. Developers ask “what will detention cost?” and budget a number. The invoice is the smaller problem.

Take the illustrative 800 square metre site. Nineteen cubic metres of storage, constrained to 200 millimetres in hardstand or 600 millimetres in landscaped areas, cannot sit in courtyards or overland flow paths. So it consumes yard, a parking space, or a strip of the site you had allocated to something else. Add the freeboard requirement — 500 millimetres above the 1% AEP flood level in Penrith's case — and floor levels may rise, reducing achievable height and forcing split-level or basement solutions that cost far more than the tank.

Meanwhile the site that cannot bypass more than 15% of its area has to detain almost everything, which can mean the difference between two dwellings and one, or between a driveway you can use and one you cannot.

Cost, timing and feasibility implications

• Land take. Storage volume divided by permitted depth gives you a footprint. That footprint competes directly with dwellings, parking and private open space.
• Design cost. Councils require a stormwater concept plan, calculations and certification with the application. Penrith warns that failure to provide a satisfactory plan “may result in your application being rejected or refused”. Bayside requires a flood study or overland flowpath assessment signed by an engineer declaring it follows Australian Rainfall and Runoff and the NSW Floodplain Development Manual.
• Floor levels. Freeboard requirements lift habitable floor levels above the design flood level, which changes the building, not just the plumbing.
• Title encumbrance. A positive covenant on title is a permanent feature of the product you are selling, and it affects the buyer's future maintenance obligations.
• Program. Flood studies and engineering certification sit on the critical path to lodgement, and a Sydney Water compliance certificate is generally needed before an occupation certificate.

The Think Property Club lesson

Stormwater teaches the discipline of asking a constraint to show you its land take, not just its price. On any small site, the question “how much of this block does it consume?” is the one that decides feasibility, and it can be answered before the offer with a catchment map, a flood level enquiry and a storage calculation.

That is repeatable, evidence-based due diligence — and it is exactly the check a capable student runs before falling in love with a yield figure.

Your next actions before you make an offer

• Order a section 10.7 planning certificate and a council flood level enquiry for the address.
• Check whether the lot sits in a mapped detention catchment or is served by a stormwater asset that requires approval to connect.
• Get the permissible site discharge and site storage requirement in writing, with the site areas supplied.
• Calculate the land take at the permitted storage depth, and test it against your layout.
• Walk the site for overland flow: upstream catchment, sag point, lowest point, discharge point.
• Confirm the discharge capacity at the connection point, and whether any boundary fence over a flow path must be open-type.
• Ask your solicitor about the practical effect of a positive covenant on the title before you resell.

Reader question: if your stormwater storage had to sit at 200 millimetres deep, where exactly on your site would it go?

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