Rainwater tanks can be an important part of BASIX water compliance for NSW residential projects. They may help reduce potable water demand by supplying alternative water for uses such as garden irrigation, toilet flushing, laundry or pool top-up, depending on the project and how the tank is connected.
A rainwater tank should not be treated as a generic note added at the end of the plans. BASIX may rely on the tank capacity, connected roof catchment area and nominated water uses to meet the water target. If the BASIX Certificate includes a rainwater tank commitment, the tank should be practical for the site and shown consistently in the plans, plumbing design or project specifications.
Short answer
Rainwater tanks are needed for BASIX when the project requires alternative water to meet the BASIX water target. BASIX can assess tank capacity, roof catchment, connected uses and pool or spa top-up arrangements. If the certificate relies on a rainwater tank, that commitment should match the plans and be realistic for the site.
Why rainwater tanks matter in BASIX
BASIX includes a water section for NSW residential development. The purpose is to reduce potable water demand compared with pre-BASIX residential water use. NSW Planning explains that a residential development proposal is required to use up to 40 percent less potable water than the average pre-BASIX home benchmark, and that the water target varies across NSW climatic zones. NSW Planning Portal
Rainwater tanks are one way to help achieve this water outcome. BASIX recognises rainwater as an alternative water source collected from roof areas and stored in a rainwater tank. Other recognised alternative water sources include stormwater, greywater, recycled effluent and, for single dwelling developments only, private dams. NSW Planning Portal
How BASIX assesses rainwater tanks
BASIX assesses rainwater tanks based on practical design inputs. NSW Planning explains that BASIX assesses rainwater based on the capacity of the tank being installed in litres, the area of roof catchment connected to the tank and the uses to which the rainwater is allocated. NSW Planning Portal
This means the BASIX outcome can be affected by:
- Tank capacity in litres.
- Roof catchment area connected to the tank.
- The number and type of water uses connected.
- Whether the rainwater supplies toilets, laundry, garden irrigation or pool top-up.
- Whether the tank is individual or part of a central water system.
- Whether the tank is new or existing.
Does every BASIX project need a rainwater tank?
No. Not every BASIX project will necessarily need a rainwater tank. Some projects may meet the water target through other measures, depending on the project type, location, water target zone, fixtures, landscaping assumptions, pool or spa details and other inputs. Other projects may need a rainwater tank to meet the required water outcome.
The important point is that the rainwater tank should be assessed as part of the full BASIX water strategy. It should not be added without checking whether there is enough roof catchment, whether the tank location works and whether the nominated uses can realistically be connected.
Tank size and roof catchment area
Tank size matters, but it is not the only factor. A large tank with very little roof catchment may not perform as expected because there may not be enough connected roof area to fill it. A smaller tank with a strong roof catchment and appropriate connected uses may perform differently in the BASIX water calculation.
This is why the assessor needs both the tank volume and the roof area connected to the tank. The site plan, roof plan, downpipe locations and plumbing concept can all help confirm whether the BASIX rainwater commitment is realistic.
Where should a rainwater tank be located?
Rainwater tank location should be considered early because it affects catchment, connections and site planning. NSW Planning explains that rainwater tanks should be located where the connected roof area and connections to fixtures can be maximised, and that downpipe locations can help confirm where the best tank location would be and how much roof area can be connected. NSW Planning Portal
In practice, the tank should fit with the site layout, setbacks, landscaping, access, plumbing and visual design. If a tank is committed in BASIX but there is no practical place to install it, the project may face problems during certification or construction.
What can rainwater be connected to?
Rainwater tanks can be allocated to different uses depending on the project and BASIX pathway. These may include toilet flushing, laundry use, private garden watering, common landscape irrigation or pool and spa top-up. The nominated uses should be realistic and supported by the plumbing design.
For pools and spas, NSW Planning states that only water from individual rainwater tanks can be nominated as the supply for pool or spa top-up in BASIX. If rainwater is used for pool or spa top-up, a tap connected to the rainwater tank must be located within 10 metres of the pool or spa edge. NSW Planning Portal
Can an existing rainwater tank be used for BASIX?
An existing rainwater tank cannot always be used for a new BASIX commitment. NSW Planning explains that rainwater tank requirements on a BASIX Certificate indicate the required additional alternative water to meet the requirements of the proposed new development, and that it is generally not possible to utilise an existing water tank because that water is already allocated to existing uses. NSW Planning Portal
In some cases, a larger existing tank may have spare capacity that can support new development demand, such as a new pool or secondary dwelling. However, this needs to be demonstrated to the consent or certifying authority. The project team should not assume an existing tank can be used without checking the BASIX requirements and approval pathway.
Central water tanks in multi dwelling projects
For multi dwelling projects, a central water tank may be used. NSW Planning explains that a central water tank can collect rainwater or stormwater and may supply common area uses, such as landscaping, car washing bays or cooling towers, or private uses for individual dwellings, such as garden watering, toilet flushing or laundry. NSW Planning Portal
Central systems need careful documentation because the tank may serve more than one dwelling or common area. The BASIX assessment may need to understand collection areas, tank volume, overflow arrangements and the uses to which water is allocated.
Rainwater tanks and council stormwater requirements
BASIX rainwater commitments should not be confused with council stormwater detention or retention requirements. A local council may require stormwater management for drainage or flood control, while BASIX is focused on household water reuse and reducing potable water demand.
NSW Planning explains that where stormwater tanks are used, BASIX is only concerned with the portion of the stormwater tank available for household reuse, and that additional volumes specified by council for onsite detention must be in addition to the volume nominated as available for household storage and reuse in BASIX. NSW Planning Portal
Can changing a rainwater tank affect a BASIX Certificate?
Yes. Changing a rainwater tank can affect a BASIX Certificate if the tank is part of the water commitments. This includes changing the tank size, removing the tank, changing the connected roof area, changing the uses supplied by the tank or altering a pool top-up arrangement.
If the rainwater tank commitment changes after the BASIX Certificate is issued, the certificate should be checked before the project relies on it for approval, construction certification or installation. Some changes may be acceptable, while others may require an updated BASIX assessment.
What information does the assessor need?
To assess rainwater tanks properly, the assessor needs enough information to understand the tank, catchment and connected uses. If these details are not yet confirmed, the BASIX water outcome may need to be reviewed again once the design is updated.
Useful information includes:
- Proposed rainwater tank capacity in litres.
- Site plan showing tank location.
- Roof plan showing catchment area connected to the tank.
- Downpipe locations, if available.
- Nominated rainwater uses, such as toilet flushing, laundry, garden irrigation or pool top-up.
- Pool or spa details, if rainwater top-up is proposed.
- Whether the tank is new, existing, individual or central.
- Any council stormwater or onsite detention requirements.
How Certified Energy can help
Certified Energy prepares BASIX Certificates for NSW residential projects and can review how rainwater tank commitments affect the BASIX water outcome. Our team can help identify whether a tank is likely to be required, what information is missing and whether the proposed tank commitment aligns with the plans and project scope.
This is especially useful for new homes, alterations and additions, secondary dwellings, dual occupancies, townhouses, multi dwelling projects and pool or spa projects where water commitments need to be coordinated before lodgement.
Need help with rainwater tanks and BASIX?
Send your plans and water system details to Certified Energy and our team can review how rainwater tank commitments may affect your BASIX Certificate.
Request a ReviewRelated resources
- Complete Guide to BASIX Certificates in NSW
- BASIX Certificate Service
- BASIX for Swimming Pools and Spas in NSW
- Does My BASIX Certificate Need to Match My Plans?
- What Documents Do You Need for a BASIX Certificate?
Frequently asked questions
When are rainwater tanks needed for BASIX?
Rainwater tanks may be needed for BASIX when a residential project requires an alternative water source to meet the water target. BASIX assesses rainwater based on tank capacity, roof catchment area, connected uses and how the rainwater is allocated.
How does BASIX assess rainwater tanks?
BASIX assesses rainwater tanks using the capacity of the tank in litres, the area of roof catchment connected to the tank, the uses supplied by the tank and whether rainwater is used for garden irrigation, toilet flushing, laundry, pool top-up or other nominated uses.
Can I use an existing rainwater tank for BASIX?
An existing rainwater tank cannot always be used for a new BASIX commitment because its water may already be allocated to existing uses. In some cases, a larger existing tank with spare capacity may be considered, but this needs to be demonstrated to the consent or certifying authority.
Can changing a rainwater tank affect a BASIX Certificate?
Yes. Changing the rainwater tank size, location, roof catchment area, connected uses or pool top-up arrangement can affect the BASIX water outcome. If a rainwater tank commitment changes after the certificate is issued, the BASIX Certificate should be checked.
