Your Guide to Stormwater Hydraulics Q&A WITH A HYDRAULIC DESIGN EXPERT ON REQUIREMENTS, PROCESSES & APPROVALS
Stormwater hydraulics is a complex but essential part of building a new home, ensuring rainwater is properly managed and redirected away from the property. In new land subdivisions, stormwater infrastructure, such as mains, pits and connection points, is typically already in place. However, the developer and council may put additional requirements on each property, which means a stormwater hydraulic design may be required. In older areas where homes are being knocked down and rebuilt, these provisions often don’t exist, so a stormwater hydraulic design is usually necessary.
For acreage homes without access to town water, a stormwater hydraulic design may also be needed to determine the best way to manage and redirect rainwater on the property. It is important for you to understand how hydraulics impacts your build and what planning is required on your end. Providing key details early like driveway placement, landscaping and pool installation, can help streamline the process and avoid unexpected costs. To break it all down, Mark Ibrahim, a leading expert in stormwater hydraulic design, shares his insights on what hydraulics is, why it matters and what you need to know when building your home.
Need to know! Once you sign your Building Contract and pay the balance of your 5% deposit, we order the documents required for your building application, including your stormwater hydraulic design. Your stormwater hydraulic design cannot be finalised until several other documents are received such as your landscape plan, BASIX certificate and arborist report. Because the cost of a stormwater hydraulic design can vary depending on site conditions and expert reports, your Building Contract may include a Provisional Allowance. This is an estimated cost included to help you plan your budget. The final cost is confirmed once the hydraulic design has been completed.
Q1. Can you tell us a little bit about yourself?
I have been in the building industry for over 30 years and a chartered member of the Institution of Engineers for more than 20 years. In 2003, I founded Ibrahim Stormwater Consultants, specialising in stormwater hydraulic design for both project and custom home builders. Since then, we’ve expanded to work with a wide range of builders across NSW. Q2. What is stormwater hydraulics and why is it important when building a home? Stormwater hydraulics is all about managing rainwater, ensuring it drains properly and is safely collected and redirected away from your property. A well-designed hydraulic system does this effectively while also meeting council and NSW building requirements. Q3. What are the key parts of a stormwater hydraulic design? A stormwater hydraulic design is typically required in areas where existing homes are being knocked down and rebuilt. These areas often have stricter council requirements because the original infrastructure was designed for smaller homes with less land coverage. When a larger home is built, it increases rainwater runoff, which can put pressure on the existing system. A stormwater hydraulic design is required to ensure rainwater is properly managed and efficiently redirected away from the new home to meet council requirements. In some cases, a hydraulic design may also be required for a vacant site. This is assessed on a case-by-case basis, depending on council requirements and site conditions.
The final design must then be approved by council or your certifier.
Below are the key elements we consider when designing the best solution for your block: → Hard Surfaces – Surfaces like roofs, driveways and paved areas where water collects. → Soft Surfaces – Lawns and gardens that absorb some water naturally. → Stormwater Pits – Connects the pipe network, collects rainwater and sends it where is needs to go. → Pipes & Channels – Carries water from one place to another. → Gutters & Downpipes – Moves rain from your roof into the drainage system. → Absorption Trenches – A storage system that allows excess rainwater to slowly release into the ground. → On-Site Detention (OSD) – A temporary water storage unit that stores excess water during heavy rainfall and then releases it slowly and safely (often required by NSW councils). → Stormwater Easement – A legally designated area within a property or neighbouring property that provides access for rainwater to be redirected to a point of discharge. → Point of Discharge – The approved location where water leaves your property, like a street kerb and gutter, street drain, easement or natural watercourse e.g. a creek or dam. → Other requirements – Some homes need rain gardens or swales (shallow channels that direct, filter and absorb rainwater) to satisfy council requirements.
Call 13 63 93 | Clarendon.com.au
Clarendon Homes (NSW) Pty Limited. (ABN 18 003 892 706) |
Ground Floor, 21 Solent Circuit, Norwest NSW 2153
|
Builder Licence 2298C
|
12/2025
Q4. When should customers start thinking about hydraulics and what do they need to consider? Right from the start! Certain factors will affect your stormwater hydraulic design and costs, so planning ahead is key. It’s important to think about the following and let Clarendon Homes know your plans as early as possible. → Landscaping – Are you planning footpaths, paved areas or mostly lawn? The more hard surfaces in your initial landscape plan, the more complex and costly the hydraulic design may be. Keeping hard surfaces to a minimum helps meet council requirements and reduces costs. It’s also important to note that changing your landscape plan after your home is built can affect the design and impact certification. → Driveway placement – What size will your driveway be and where will it be located? This information is important when determining where pipes and drains need to go, it also contributes to the amount of hard surfaces. → Pools – Are you planning to install a pool? Let Clarendon Homes know early, as pools are considered a hard surface and will impact the stormwater hydraulic design, including where drainage pipes are placed. If your pool exceeds 40,000L, it will also require a BASIX Certificate and may affect your rainwater tank size, so be sure to discuss this with your pool company. Q5. What are common stormwater hydraulic design challenges? One of the biggest challenges with hydraulics is that every site is different and council requirements vary. Some councils have strict rules that can make compliance more difficult, often requiring additional design elements that increase costs. Certain properties also don’t drain well, which means extra work may be needed, such as adding absorption pits, OSD or upgrading pipes. If an easement is required to direct water through a neighbouring property, legal requirements can also add another layer of complexity . Q6. What affects the cost of hydraulics? There are several factors that can influence the cost of your home’s stormwater hydraulic design, including: → The amount of hard surfaces – one of the biggest factors that drives up cost is the amount of hard surfaces on your property. The more paved or concrete areas, the more complex the design, which increases costs. → The need for extra design elements – if additional hydraulic solutions are required, such as absorption pits, detention basins, rainwater tanks or upgraded pipes, costs will increase.
→ The slope of your block – homes on higher ground can dispose of rainwater to the kerb using gravity, allowing water to flow naturally to the street. However, homes on lower ground may require alternative drainage solutions, such as a stormwater pit, absorption trench or easement. These additional requirements can increase costs. Q7. What additional hydraulics costs can come up? There are a few unexpected costs that can come up with hydraulics, depending on your site and council requirements. Some things to be aware of include: → Council restoration fee – If a road opening permit is required to lay pipes under the nature strip, which is owned by the council, a restoration fee may apply to cover the cost of rectification work. → Easement approval – If rainwater needs to be redirected through a neighbour’s property, additional costs may apply for legal approvals, plumbing work and compensation to the neighbour. → Flood restrictions – If your council determines that your land is in a flood zone, additional reports may be required to assess whether any adjustments are needed to the stormwater hydraulic design and home, which can add to the cost. Q8. Lastly, do you have any advice about hydraulics for our customers? My advice applies to two key stages, before construction starts and after your home is complete and you receive your keys. Plan ahead and communicate your plans upfront. If you’re considering a pool, a wide driveway or large hard-surfaced areas, let Clarendon Homes know early in the process to avoid costly changes later. After handover, when completing your landscaping, give your landscaper a copy of your stormwater hydraulic design and landscape plan. It’s important that your landscaping is completed in accordance with the plans, including hard surface areas and ground levels. For example, the hard surfaces on site must match those on the approved landscape plan and the ground levels around your home should align with those specified in the stormwater hydraulic design.
Have questions?
If anything is unclear or your landscaper proposes changes, contact your Clarendon Homes Customer Service Consultant.
Call 13 63 93 | Clarendon.com.au
Clarendon Homes (NSW) Pty Limited. (ABN 18 003 892 706) |
Ground Floor, 21 Solent Circuit, Norwest NSW 2153
|
Builder Licence 2298C
|
12/2025
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