Approval of onsite wastewater systems for ancillary dwellings

The following information details the Department of Health’s requirements for onsite wastewater systems servicing ancillary dwellings in Western Australia.

What is an ancillary dwelling?

An ancillary dwelling is a dwelling for people who live either independently or semi dependently to the resident of the single house on the same lot, sharing some site facilities and services.

An ancillary dwelling must have planning and building approval from the Local Government and must meet the Department of Planning requirements.

Where can I dispose wastewater from an ancillary dwelling?

Two methods for onsite wastewater disposal are available to service an ancillary dwelling.
  • Connection to an existing onsite wastewater system serving the main house. 
  • Installing a separate onsite wastewater system.

How are the onsite wastewater systems servicing ancillary dwellings sized?

Where an ancillary dwelling is to be connected to the existing onsite wastewater system the following applies:
  • Wastewater loading will be calculated on the combined hydraulic loading based on the total combined number of bedrooms (ancillary dwelling PLUS main residential dwelling) and sized as a "Standard residential dwelling" applying the following formula:                                                                                  Wastewater loading (L/day): Number of occupants (persons/day) x 150L/person/day
  • The Local Government Environmental Health Officer (EHO) will determine the combined occupancy rates for both the main house and ancillary dwelling.
  • The receptacle for drainage shall be sized using the wastewater loading determined with the formula above. 
  • Septic tanks shall be sized according to Regulation 28 of the Health (Treatment of Sewage and Disposal of Effluent and Liquid Waste) Regulations 1974 (Regulations).
  • The EHO must be notified to determine if the existing is sufficiently sized and if an upgrade to the system is required prior to connection.


  • 3 bedroom main dwelling PLUS 2 bedroom ancillary accommodation = 5 bedroom system (829L/day)
  • 4 bedroom main dwelling PLUS 2 bedroom ancillary accommodation = 6 bedroom system (150L/person/day).

Where an ancillary dwelling has its own separate system, the system shall be sized according to the Regulations as a residential dwelling.

When will the application to construct and install the wastewater system require the approval by the Chief Health Officer?

When the combined occupancy of a single dwelling and the ancillary dwelling exceeds 8 persons, the alteration, construction and installation of any apparatus for the treatment of sewage will require approval by the Chief Health Officer (Department of Health).

If the combined occupancy of the main house and ancillary dwelling is less than or equal to 8 persons, the wastewater system serving the ancillary dwelling (either combined or separate) can be approved by local government.

When will an ancillary dwelling become a separate dwelling to the main house?

An ancillary dwelling becomes a separate dwelling when the land is subdivided and the ancillary dwelling is located on a separate lot to the main dwelling.

If this occurs both dwellings will require their own separate onsite wastewater system that complies with regulations.

Last reviewed: 16-10-2019
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Public Health