When building a brand new home in an existing suburb, you are (more than likely) going to need a hydraulics system that involves a rainwater tank and an on-site detention system (OSD).  It is important to have a good understanding around how it all works.

What is Hydraulics?

Hydraulics is about designing a system that manages a clean water supply, stormwater solution and efficient usage of the water within your block of land.  This includes, but not limited to, designing:

  • pipes / down pipes / underground pipes / etc…
  • plumbing systems
  • drainage
  • rainwater management
  • wastewater management
  • water supply
  • connection to the public system

The Who?

A Hydraulics Engineer designs your Hydraulics System for your new home build.  Your builder generally has an existing relationship with a hydraulics engineer that designs all of their system for your new home construction.

hydraulics engineer is a civil engineer who specializes in the properties and movement of liquids like water and sewage.

Hydraulic Components

There is a number of management components in your home hydraulics design.  Having said that, we will be discussing three different water management components in this blog so we can understand what they are, how they work and how they can be priced.  These three components are:

  1. Rainwater Tanks
  2. On-Site Detention System
  3. Absorption Pits

Rainwater Tanks

As the name suggests, a rainwater tank is a container that is connected to your home pipes so it can collect rainwater.  It’s an environmentally friendly way to collect rainwater so you can use it in your garden and home.

Rainwater Tanks can be bulky and not the best looking for your back or side yards.  They are usually installed by your builder and your BASIX will generally indicate the size of the tank you need for your specific home.  Some of the key considerations when it comes to rainwater tanks include, but not limited to:

  • BASIX requirements (of course)
  • Tank Size
  • Pump Type
  • Installation & Connections
  • Tank Type (above or below ground)

On-Site Detention System

An On-Site Detention System is “like” an underground rainwater tank that is used to detain stormwater from your site before pushing it, safely, to the public system.  It’s a water storage tank with a slow release mechanism in place to drain the water slowly to the council kerb or street system.

This is generally utilised in addition to your standard rainwater tank for additional water storage.  The purpose of this additional system, specially in existing knock-down-rebuild areas, is to improve drainage and avoid flooding during heavy rains.

In other words, it is there to help the public infrastructure cope as they may be old and have not been upgraded.  So council generally “makes” home owners build these additional detention systems to ease the pressure on the public stormwater system.

This method of holding stormwater runoffs temporarily before slowly releasing it slowly into the public water system.  They are different to rainwater tanks in how they are fitted and how they operate when it comes to detaining and releasing water.

Absorption Pits

An Absorption Pit or Trench is a “hole” in the ground, fitted with piping and rested with gravel and filled with topsoil.  It is up to the engineer if this type of stormwater management is needed for your site.


Hydraulics can cost anywhere between $4,000 to $120,000 subject to what and where you are building.

Fixed vs. Provisional Allowance?

So can the builder fix the cost of your hydraulics system, if it involves more than a simple rainwater tank?  Short answer is YES and they MUST.  However it’s a timing problem.

Builders cannot fix the hydraulics costs (assuming it involves an OSD tank for example) until the hydraulics engineer designs your system properly and council’s hydraulics engineer approves it.

So until then, all the builder can do is give you a provisional allowance for the potential cost of the design, engineering, construction and WAEX (works as executed).

Obtaining a fixed price is not possible during your tendering / initial stages as council must approve your hydraulics design.

What if I want it fixed upfront?

If you really want your builder to fix your hydraulics costs, you can pay for the design done and the builder can then price and include a fixed price for the specific design.  However this is still subject to final approval from your council.

If council happens to approve the proposed system as it is, then your price doesn’t change as the design does not change.  If your council does not approve it, then the builder’s hydraulics engineer will amend your proposed system and the builder will in turn update the hydraulics price.


It is important to understand and research this particular part of your build.  It is also important to understand your chosen builder’s process.  Just because one builder does things a certain way, this does not mean that the next builder will do the same.

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