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.