What is an ancillary dwelling?

An ancillary dwelling is a small, self-contained unit (kitchen, bedroom, bathroom and living area) on the same lot as a single (main) house. An ancillary dwelling can be under the same roof or detached from the main house. Occupancy is no longer restricted to family in Rural zones (state-wide State Government regulatory change February 2021).

Ancillary dwellings may be planned with a particular occupant in mind, however the Shire recommends that all ancillary dwellings be built in accordance with accessibility guidelines including wider doorways, hallways and bathrooms. This will allow for continued use if the occupant temporarily needs a wheelchair, or future use by elderly or disabled people.

What is permitted?

Ancillary dwellings in Rural zones or Special Control Areas (e.g. bushfire prone areas) require planning approval prior to applying for a building permit. In Residential zones, if the property is not within a Special Control Area and the design complies with all relevant provisions of the Residential Design Codes (R-Codes) then planning approval is not required and you would just need to apply for a building permit.

There are restrictions on the size and characteristics of ancillary dwellings. An application for an ancillary dwelling would be assessed against the provisions of the Shire of Mundaring’s Local Planning Scheme No. 4 (LPS4) and R-Codes. These are some of the points that would be considered:

  • Size of the ancillary dwelling is generally limited to 70sqm (note that this is the internal living area and excludes carports, verandas and the like). The Shire may approve up to 80sqm where the ancillary dwelling is being built under the main roof, or the additional area is to meet accessibility guidelines;
  • Location should be close to the main house and comply with boundary setbacks;
  • Appearance of the ancillary dwelling should be very similar to the main house - match the roofline and roof materials; use similar wall colours, window frames, eaves and verandas.

Situations where ancillary dwellings may not be approved

Ancillary dwellings are a discretionary use in most zones. This means that the proposal will be assessed on its merits and approved if it is appropriate.

Some circumstances that may prevent or limit ancillary dwellings are:

  • If there is no sewer connection, then lot size and capacity for on-site effluent disposal may limit the development or size of the ancillary dwelling;
  • If the building would be in an environmentally sensitive area, extreme bushfire hazard area, flood prone area or public drinking water catchment it may not be approved;
  • If the building does not comply with LPS4 or R Codes requirements (too large, too far away from the main house, or the appearance is not similar to the main house).

Locating ancillary dwellings

On Residential zoned lots the setbacks should be in accordance with the R-Codes (Table 1 and Table 2). On larger Residential lots the ancillary dwelling should be located reasonably close to the main house.

Part 5 of the Shire’s LPS4 provides for setbacks for dwellings in various zones. On Rural Residential and Rural Small Holdings zoned properties, the building should be within the building envelope if there is one, or in a cleared area in accordance with the setbacks for a dwelling for that zone. Where possible, a 20m setback from boundaries is recommended so that the landowner can manage the 20m area surrounding the dwelling as a ‘low fuel zone’ to reduce bushfire risk.

Wastewater disposal on unsewered lots

In some cases the ancillary dwelling will be able to connect to the existing effluent disposal system for the main house. If this is not possible, a second system may need to be installed.

An application for a second effluent disposal system is still made to the Shire but will be referred to the WA Department of Health for approval. 

Find out more about effluent disposal systems.

Apply for planning approval and building permit

Step 1. Apply for planning approval

To make an application for planning approval you will need to provide details of the site and existing and proposed buildings. An application must include: 

  • Completed planning application form (Form 1) signed by all owners and planning application fee (refer to current Shire Schedule of Fees);
  • A current copy of the Certificate of Title (less than 6 months old)
  • A scaled site plan showing lot boundaries, all existing buildings, existing and proposed effluent disposal systems and the precise location of the proposed ancillary dwelling
  • Floor plans of both the main house and proposed ancillary dwelling
  • Elevations of proposed ancillary dwelling showing materials, wall height and ridge height, natural ground levels, earthworks and method of retaining (note that the appearance of ancillary dwelling must be similar to the main house, or include modifications to the main house to match the new ancillary)
  • If the lot is within LPS4’s Bushfire Hazard Special Control Area, include a Bushfire Attack Level and a Bushfire Management Statement.

Step 2. Apply for a building permit

A separate building permit application will need to be submitted to the Shire’s Building Services for approval, prior to construction and after planning approval is issued.

The application for the building permit will require structural details and engineering certification. Some planning conditions may need to be met before the building permit can lodged and/or issued.

Building works valued under $20,000 do not require a registered builder allowing owners to nominate themselves as “builder” on the building application form.

For building works over $20,000 a registered builder must be nominated as the builder. Owners who wish to appoint themselves as the builder for building works over $20,000 are required to attain an owner-builder certificate from the Department of Mines, Industry Regulation and Safety.

Owners who have chosen to do the earthworks in lieu of their builder, must obtain a separate building permit prior to any earthworks commencing.

Complete application form

You will need to complete a Application for building permit – uncertified (BA2).

Ensure you include: 

  • Architectural drawings including:
    • Site plan
    • Floor plan
    • Elevations
    • Sections
    • Electrical plans
  • Energy Efficiency Report
  • Site classification - issued or certified by a geotechnical engineer
  • Engineering - all structural and construction details designed and certified by a registered structural engineer
  • BAL construction specifications and Bushfire Attack Level (BAL) Assessment and Certificate (if located in a bushfire prone area)
  • Termite management details
  • General specifications
  • Home Indemnity Insurance (for residential works valued over $20,000)
  • Stormwater Drainage Plan.

Refer to Building Permit Fee and Calculator.


Building permit applications either certified or uncertified are usually assessed and approved within 10 business days.

Building permits are sent to the builder via post.

    Step 3. Submit a Notice of Completion

    Under the Building Act 2011, the builder must notify the Shire that the work is completed by submitting a BA7 Notice of Completion Form.

    More information and contact

    For more information about planning approval, contact Planning Services on (08) 9290 6740.

    For more information about building permits, contact the Building Services on (08) 9290 6666

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