Using an existing rainwater tank to fulfil BASIX requirements

Rain water tank requirements on a BASIX certificate indicates the required ADDITIONAL alternative water to meet the requirements of the new development proposed. The aim is to reduce potable water demand.

For an alteration or addition to an existing dwelling, such as installing a new swimming pool, the water demand arising from the new development is estimated and the commitment on a BASIX certificate shows the water efficiency requirements, such as a rainwater tank, required to satisfy the additional water demand.

It is generally not possible to utilise an existing water tank, as that water is allocated to existing uses and its allocation to a new use will not achieve the aim of the BASIX policy.

However, in some cases, where there is a larger than normal rainwater tank already existing on a site that has 'spare' capacity, it may be possible to demonstrate that the additional water demand generated by a new pool or secondary dwelling can be met by the existing tank without any adverse effect on potable water demand.

Determining if this is the case is a matter for the consent authority when considering the environmental impacts of the development.

If you believe there may be adequate capacity in the existing tank to serve both the existing dwelling and the new development, the following guide can be used and the documented outcomes submitted to the consent authority for their assessment.

As a general guide, if the existing tank is less than 4,000 to 6,000L, it is unlikely to have spare capacity. For an average suburban house and lot, the existing tank would typically need to be more than 7,500L to cater for both existing and new uses and for a large rural lot, the existing tank would need to be significantly larger, typically 20,000 litres or more.

Method to ascertain if an existing tank is of adequate size

Where a BASIX Alterations and Additions Certificate or a BASIX Single Dwelling certificate for a new secondary dwelling nominates the installation of a new rainwater tank, but an existing tank already exists on site, the following guideline may be used to determine if the existing tank has adequate capacity to serve both the existing and new uses.

This guidance only applies to certificates generated using the New Dwellings Singles tool, or the Alterations and Additions tool. Apportioning alternative water supply in the multi-dwelling tool is only applicable for new developments using common or shared services.

The purpose of the process is to demonstrate to a relevant approval or certifying authority that the size of the existing tank has sufficient capacity to supply the necessary volume of alternative water to both the existing dwelling and any new part or dwelling subject to the BASIX certificate.

  1. Generated a BASIX Certificate (A) describing the proposed new development, such as an alteration and addition to the existing house, a new swimming pool, or new secondary dwelling. The certificate should be in accordance with the standard requirements for these types of BASIX effected development. This certificate will show the minimum size of a rainwater tank required to satisfy the BASIX requirements to supply alterative water supply applicable to the proposed new development.

  2. Create a BASIX Report (B) that describes the status of the existing dwelling in the water section of BASIX. This report should not include the proposed new works. It should fully describe:
              In the Project Details section
    1. the number of bedrooms for the house, not including any additional bedrooms created in the proposed new development (example: 4 bedrooms);
    2. the site area of the lot, minus any of the site allocated to the proposed new works described in Certificate A above (example: 700m2 less 80m2 = 620m2)
    3. the roof area of existing structures (house, garage etc) not counting any proposed new roof area, (example: 250m2);
    4. the conditioned and unconditioned floor area of the existing dwelling (example: 210m2); and
    5. existing pools or spa, not including any proposed new pool or spa included in Certificate A;
      In the Water section:
    6. the landscape area (garden and lawn) and any component that is planted with indigenous or low water use species, not including any landscaped area allocated to the new development proposal and shown on Certificate A (example: 310m2);
    7. water efficiency (WELS) ratings for existing water fixtures in the dwelling (WC, taps, shower heads etc). Where the WELS rating of a fixture is not known or cannot be verified, the lowest available WELS rating should be selected from the BASIX menu;
    8. nominate the existing alternative water supply(s). Only include systems such as greywater or private dams where they are used to also supply alternative water to the existing dwelling;
    9. the area of existing roof (house, garage etc) allocated to supply the rainwater tank, not counting any proposed new roof area, or any existing roof areas now allocated to supply alternative water in Certificate A (example: 160 m2 less 40m2 of existing roof allocated to supply water to the tank shown in Certificate A = 120m2);
    10. the size of the existing tank LESS the volume (in litres) stated on Certificate A as required to fulfil the BASIX commitments for the new development, (example: 10,000L tank less 1,500L allocated to new development = 8,500 allocated to existing dwelling); and
    11. less any volume of water set aside under other regulation or council requirement for purposes such as firefighting or bush fire control; and
    12. details for any existing pool or spa, not including any proposed new pool or spa included in Certificate A.

  3. The BASIX report for the existing dwelling should show that the existing rainwater tank can reach or exceed the required BASIX water target with a volume of water NOT INCLUDING the volume of water proposed to be allocated to the new development. For example, 7,500L is required for existing house, leaving 2,500L ‘spare’.

  4. A copy of the report should be saved and submitted to the consent or certifying authority to demonstrate that the existing tank is of adequate size to meet the BASIX water requirements of both the exiting dwelling and the proposed new development.

CHECK: The sum of each area (site area, landscape area and roof area allocated to supply the water tank ) entered in both Certificate A (for the new development) and the BASIX report B (for the existing dwelling) should not exceed the totals for overall site area, landscape area or roof area – that is, there should be no ‘double counting’.


 Example1 existing water tank


 Example2 existing water tank



Use of existing water tank in multi-dwelling projects

The multi-dwelling tool allows certain facilities and services, such as a shared rainwater tank, to be nominated as shared between individual dwellings. The software then apportions the water use between the individual and common areas and confirms whether sufficient alternative water is available to reach the required BASIX water target.

In this case, the common tank would be a new water tank specific to the described development and not form part of any existing common services.

Potential hazards and treatment

To minimise detrimental impacts on rainwater quality, potential water pollutants should be intercepted prior to draining into the tank. Discoloured rainwater can be of concern to some for washing clothes and toilet flushing. Odours from poor quality rainwater due to breakdown of leaves and other organic matter in the tank can also be undesirable.

Below are a few options.

Screening  Screening devices positioned between the roof surfaces and the rainwater tank assist to separate this debris from the rainwater.

Combinations of coarse and fine mesh screens can be provided to remove different size debris and prevent tank access by mosquitoes, frogs, rodents and other animals.

Screens are typically provided at breaks in downpipes.

Coarse and fine mesh screens are usually provided in the roof of the rainwater tank to filter rainwater at inlets.

Finer screens are provided over outlets from the tank to prevent mosquito access.

Diagram showing screen at break in downpipe

Diagram showing coarse and fine mesh screens in the roof of the rainwater tank to filter rainwater at inlets.

First Flush Diverters  A first flush diverter should be provided prior to a rainwater tank inlet and after a screening device.


 First flush devices should be sized to divert the first flush of rainwater from the roof which typically contains the highest concentration of pollutants.

Diagram of a diverter

Passive Treatment

Passive water treatment occurs within the tank due to settling of sediments and formation of biofilms on the internal walls of the tank. To prevent excessive build-up of sediment, dewatering of the rainwater tank should occur every few years depending on location. A valve located at the base of the tank assists with completing this efficiently.

Sun-shades should be positioned under screens located in the roof of the tank to minimise light penetration into the rainwater tank to restrict the growth of algae and other unwanted biological activity in the tank that can lower the rainwater quality. Rainwater tanks should be positioned in the shade where possible. Sun-shades should be used to protect tanks from direct sunlight as direct sun can diminish the tank itself and light penetration into the tank can cause algae and other bacterial growth in the water stored in the tank.


 In most circumstances, screens and first flush diverters will be sufficient for providing rainwater of an appropriate quality for all non-potable rainwater uses. In circumstances where finer pollutants or discolouration of water is of concern, filters may be required. Although, filters can regularly become blocked by fine sediments requiring increased maintenance and energy input to pump water. If intending to connect rainwater tanks to all household uses, refer to the NSW Health guidelines for further information on filters and other advanced water quality treatment considerations (http://www.health.nsw.gov.au/environment/water - NSW Health website).

Size and material of rainwater tanks


BASIX requires the applicant to enter in the rainwater tank size. It is recommended that the available rainwater storage is kept to a maximum by installing any drinking water supply top-up valves within 300mm from the base of the tank. If you require a greater top-up level, it is recommended that this volume is provided in addition to the size nominated in your BASIX commitments.

Local council authorities may require rainwater storage tanks for the sole purpose of fire fighting. This cannot be included in BASIX. The rainwater tank size nominated in BASIX must be in addition to this requirement.

When determining the size of a rainwater tank the following considerations should be made.

What needs to be considered  Appropriate action

 Roof area
  • Rainwater for irrigation, toilets and washing machines can be harvested from most roof materials. Consideration should be made to the type of paint used on roofs as lead based paints should not be used for rainwater harvesting.
  • Before selecting a rainwater tank it is important to work out how much roof area can realistically be connected to the tank.
  • A conceptual roof drainage plan should be prepared to identify the locations of ridges, gullies, gutter high points and optimal downpipe locations.
  • The larger the connected roof area, the greater the potential for capturing rainwater.
  • Roof areas should be kept clear of overhanging vegetation and tree branches to minimise the quantity of leaves falling on the roof and restrict access by animals including rodents, cats and possums.

Available Land Size
  • Consideration should be made to the land size available and the potential position for a tank, keep in mind required setbacks from fencing and adjacent windows.

Water Demand
  • Consider your total water demand based on the potential uses for the captured rainwater:
    • Irrigation
    • Laundry
    • Toilet Flushing
    • All hot water use
    • Drinking and household use
Local Rainfall
  • Use local rainfall data to help determine the rainwater capture potential of your rainwater harvesting system.

If you enter rainwater tank larger than a maximum allowable size, BASIX will advise you to consider the selection by not allowing certificates to be generated. Increasing the size of an already large tank may not result in proportionate improvements in potable water savings. It may be more cost effective to consider other water efficiency measures than increasing the rainwater tank indefinitely.

You can enter the maximum allowable size as advised by the BASIX tool, but still install a larger tank on site. Since the BASIX certificate states that "The applicant must install a rainwater tank of at least [number] litres on the site", the larger tank is regarded as over-complying with the BASIX requirement.


There are a variety of rainwater tanks types available and due consideration should be made when selecting the appropriate tank type for each site.

  • Metal/Steel Tanks
  • Concrete Tanks
  • Plastic Tanks with Plastic Liners
  • Fibreglass Tanks
  • Bladders

If a rainwater tank will be located in a bushfire prone area, the tank should be fabricated from steel, concrete or other suitably protected material in accordance with NSW Rural Fire Service guidelines (http://www.rfs.nsw.gov.au/plan-and-prepare/building-in-a-bush-fire-area - NSW RFS website). Durability of other components should be confirmed with suppliers and appropriate warranties obtained for tanks and other system components.

Measuring area diverted to rainwater

The roof area that is diverted to a rainwater tank can include all roof surfaces measured to the outside of the gutters, excluding parapets and trafficable roof terraces and measured in the horizontal plane.

Roof areas diverted to a stormwater tank can include all roof surfaces (including trafficable roof terraces/decks, etc), as well as impervious areas, garden and lawn areas and planter boxes.

If rainwater is mixed with stormwater then the mixed water source is defined as stormwater and there are limits to how it can be used without treatment.

What are impervious areas?

Impervious areas are surfaces that do not allow natural infiltration of rainfall to the underlying soil and include concrete, gravel and timber surfaces.

Diagram showing how to measure area diverted to rainwater or stormwaterDiagram showing how to measure area diverted to rainwater or stormwater


Note: If you have nominated a large roof area (approximately more than 150m²) to carry water to your rainwater or stormwater tank, ensure that the piping used to carry the amount of water collected by the roof area is appropriately sized for the collection area you have nominated. Speak to your hydraulic designer/plumber for more information, as this design guide varies across NSW.

Rain water use in pool or spa

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Installation of rainwater tanks

  • The constraints of a particular site should be identified and considered well in advance of the installation. Rainwater tanks should be located where the connected roof area and connections to fixtures can be maximised.

  • After identification and consideration of the constraints, potential rainwater tank locations can typically be identified on a property constraints plan.

Example of a property constaints plan identifying potential locations of rainwater tanks


  • The downpipe locations will assist with confirming where the best location for a tank would be and how much roof area can be connected.

  • Where possible the tank should be located in a well shaded area of the property.

  • The weight of a rainwater tank when full will be vastly greater than its weight during installation (empty), it is important to ensure a solid foundation and that any nearby fences or posts will not be burdened by the total potential weight of the tank.

  • Ensure enough access to the rainwater tank for future maintenance


Diverting overflow

Water tanks often overflow in periods of high rainfall. The overflow from a water tank can be diverted to another tank for collection and use. Here you can nominate if the overflow from your tank will be diverted to another water tank, for example a stormwater tank, for collection and re-use.

Cross section of rain water tank overflowing into an underground storm water tankCross section of rain water tank overflowing into an underground storm water tank