Stormwater Design for Duplex NSW: Shared Drainage Systems Explained

Stormwater design for a duplex in NSW can be more difficult than people expect. There are two homes on the same block. Each one has its own roof, driveway, and paved space. But there is just one drainage system for everything. If you plan everything the right way from the beginning, it helps you avoid extra changes and can save you both time and money in the long run.

At R I Eng Pty Ltd, we work with both types of drainage many times. Here is what happens during the work and why the work matters for your job.

Why Duplex Drainage Design Differs from a Single House

A single home connects to the street at just one spot. A two-unit home is not like this.

Homes like these often use the same pipes for water from rain below the ground. When two homes share drains for rain, the plan for a duplex in NSW must include water that comes off both roofs, the driveway both homes use, and the water that can be held on the land. It must cover the whole area, not just one home. If someone tries to build one first and the other one after, it can cause trouble. This way does not pass council checks most of the time and can lead to problems with water not moving properly on the land.

The engineer has to look at the site as one big system from the very start.

What R I Eng Pty Ltd Designs for a Duplex

Our dual occupancy stormwater design in NSW takes care of every part that the council and certifiers want to see.

Roof catchment design — We take the roof area from both of the homes and add them together. If the roof area is big, there will need to be bigger pipes and pits. We use DRAINS software to make sure the size of these is right.

Shared OSD system — Most NSW councils ask for on-site detention when there is a duplex. We make one OSD tank for the whole site, not two smaller tanks. We count the detention volume for both homes together.

Inter-allotment drainage — When there is one house up on a hill and another lower down, we set up the inter-allotment drainage to move water from the higher place to a common spot where it flows away. This helps so the house below does not get extra water from above.

Torrens title duplex drainage — If the duplex is being divided into two lots under the Torrens title, each lot needs its own connection and drainage system. We set this up at the start, so there is no problem later when the split happens.

OSD outlet and overflow — The outlet is what helps control the speed of water flowing out of the OSD tank. We make sure it is put in right so it matches the allowed discharge rate for the site. The overflow path is there to show where water will go if the system gets full in a big storm.

Duplex Hydraulic Design: What Changes by Council

The way you design a duplex water system in NSW depends on the local council area. Parramatta, Blacktown, and Hills Shire each use different OSD numbers. A design that works for one council might not fit another. R I Eng Pty Ltd has the latest DCP numbers for all the big NSW councils. We always use the right numbers from the start. No one has to guess.

Some councils want the pipes that connect the two homes to be listed on their own as part of the total shared stormwater plan. We add this to the plan when needed.

5 Questions About Duplex Stormwater Design NSW

Q: Does a duplex need one OSD tank or two?
Most of the time, one tank is enough for the site. This makes it less costly. But there are some councils that ask for a different setup. R I Eng Pty Ltd looks into this before they get started on the design.

Q: Who installs the shared drainage system — both owners or one?

The builder does this job because it is in the building contract. R I Eng Pty Ltd gives the right engineering drawings so the builder can do the work the right way.

Q: Can the stormwater design change if we Torrens title subdivide later?

Yes, it can change a lot. You should plan the way water goes away with the idea of splitting the land from the start. R I Eng Pty Ltd can think about this for you early on.

Q: Does duplex stormwater design cost more than a single home?
Yes, it does cost a bit more. This is because there is more work for engineers. The way these homes deal with rainwater takes more steps. There is a shared system that slows down the water when it runs off. The plans also need to work for two homes, and also take into account each person’s land. If you want a simple price, contact us.

Q: How long does stormwater design take?
Most stormwater design plans will be ready in three to four business days after you send your survey data.

Talk to R I Eng Pty Ltd About Your Duplex

If you are going to build two homes in NSW and you need help with stormwater design, R I Eng Pty Ltd can help you.

📞 Call us or go to riengineering.com.au to talk with us. Tell us where your site is, which council covers the area, and what you want to build. We will let you know what you need and get back to you fast.