What are onsite effluent disposal system requirements?
Where a network wastewater system is not available or does not have sufficient capacity, a granny flat may use an onsite effluent disposal system (for example, a septic tank system), provided all exemption conditions are met.
For the purposes of the granny flats building consent exemption:
- an onsite effluent disposal system must be designed, installed and operated to comply with an existing AS or VM e.g. G13/VM4, designed to AS/NZS1547:2012
- the system must be appropriate for the site conditions and the expected loading from the granny flat e.g. 30 fixture units.
- if an existing onsite system is proposed to be used or modified, it must have sufficient capacity and condition to service the additional dwelling. This may require a review of an existing building consent of resource consent.
Compliance may involve assessment against or other relevant technical standards, as appropriate.
Regional council rules relating to wastewater discharges to land continue to apply. The building consent exemption does not override regional plan requirements or any resource consent requirements for effluent disposal. Any separate approvals required under regional rules must still be obtained.
Does an onsite effluent disposal system need to be a new one or can an existing system be used or modified?
An onsite effluent disposal system does not need to be a new system. An existing onsite effluent disposal system may be used or modified, provided it can be demonstrated that the system:
- has sufficient capacity to service the granny flat, and
- is in a suitable condition to continue to operate effectively.
Under the building consent exemption, the owner is responsible for ensuring that any onsite effluent disposal system used for a granny flat complies with the Building Code. Where an existing system is relied on, the owner is responsible for obtaining appropriate professional advice or assessment to demonstrate that the system has adequate capacity and is suitable for continued use. This may include assessment against G13/VM4, AS/NZS1547:2012.
Because this is a building consent exemption, councils:
- do not approve or certify onsite effluent disposal systems for granny flats, and
- are not required to assess or verify system capacity or condition.
Any information prepared to demonstrate capacity and condition forms part of the owner’s due diligence and project documentation.
Regional council rules relating to wastewater discharges to land continue to apply. The building consent exemption does not override regional plan requirements or any resource consent requirements for onsite effluent disposal.
How can the capacity and condition of an existing onsite wastewater system be confirmed under NZS1547?
Under the granny flats building consent exemption, under G13/VM4, AS/NZS 1547:2012 is used as a recognised technical standard to assess whether an onsite wastewater system is suitable, rather than as an approval or certification process.
Confirming capacity and condition under AS/NZS 1547:2012 means that a suitably qualified professional assesses whether the existing onsite wastewater system:
- Has capacity to service the additional wastewater loading from the granny flat, and
- is in a condition that allows it to continue operating effectively.
The outcome of an AS/NZS1547:2012 based assessment forms part of the owner’s evidence that the exemption conditions are met and supports the owner’s obligation to ensure Building Code compliance.
Because this is a building consent exemption:
- councils do not assess or certify onsite wastewater system capacity or condition under AS/NZS 1547:2012, and
- councils may rely on the owner and their professionals to ensure appropriate assessment has been undertaken.
Information may be requested by a council if compliance is later questioned, or through other processes (for example, regional council requirements), but it is not a pre‑approval step under the Building Act exemption.
Regional council rules relating to wastewater discharges to land continue to apply. Any resource consent or discharge authorisation requirements under regional plans must still be met, regardless of the building consent exemption.
How can the capacity and condition of an existing onsite wastewater system be confirmed under NZS1547?
Under the granny flats building consent exemption, under G13/VM4, AS/NZS 1547:2012 is used as a recognised technical standard to assess whether an onsite wastewater system is suitable, rather than as an approval or certification process.
Confirming capacity and condition under AS/NZS 1547:2012 means that a suitably qualified professional assesses whether the existing onsite wastewater system:
- Has capacity to service the additional wastewater loading from the granny flat, and
- is in a condition that allows it to continue operating effectively.
The outcome of an AS/NZS1547:2012 based assessment forms part of the owner’s evidence that the exemption conditions are met and supports the owner’s obligation to ensure Building Code compliance.
Because this is a building consent exemption:
- councils do not assess or certify onsite wastewater system capacity or condition under AS/NZS 1547:2012, and
- councils may rely on the owner and their professionals to ensure appropriate assessment has been undertaken.
Information may be requested by a council if compliance is later questioned, or through other processes (for example, regional council requirements), but it is not a pre‑approval step under the Building Act exemption.
Regional council rules relating to wastewater discharges to land continue to apply. Any resource consent or discharge authorisation requirements under regional plans must still be met, regardless of the building consent exemption.
Does a BCA need to confirm regional rule requirements around discharge to ground e.g. with respect to effluent discharge, should the BCA insist on a design at application stage to show compliance?
Under the granny flats building consent exemption, BCAs are not responsible for confirming compliance with regional council discharge‑to‑ground rules.
Regional wastewater requirements remain the responsibility of the owner and the relevant regional council, and BCAs should not require wastewater design information as part of administering the building consent exemption.
A BCA may:
- clearly advise owners that regional rules still apply, an
- direct owners to the relevant regional council for advice on effluent discharge requirements.