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A lot of people are looking at their block and thinking, “Could I rent out a granny flat and earn some extra income?” And honestly, it’s a smart question. Planners call these smaller second homes ancillary dwellings and they have become a really flexible way to house family, support ageing parents, or provide an affordable rental for someone who needs it.

But before you start sketching floor plans or calling the real estate agent, it’s worth understanding exactly what the Shire of Mundaring does (and doesn’t) allow. The rules are pretty clear once you break them down.

How Big Can I go?

Most ancillary dwellings are capped at 70m² of internal habitable floor area. Given that the average Australian master bedroom is 12-16m², that’s really quite big. However, you can stretch that limit to 80m² if you design your ancillary dwelling to be accessible. To qualify for the larger size, it needs to include features such as:

  • 820mm minimum clear door openings
  • Internal corridors at least 1200mm wide where there are openings on the side walls
  • A visitable toilet, ideally within a bathroom
  • A 250mm nib wall on the handle side of the toilet door, with capacity for grab rails

These elements make the home more comfortable for seniors, visitors using mobility aids, and long‑term accessibility.

Can I just buy a granny Flat online?

When designing your ancillary dwelling you should strive to use materials, colours and styles that complement the main residence, especially on properties listed on the Shire’s Local Heritage List / Survey. The ancillary dwelling should be located as close as reasonably practicable to the main residence, while balancing privacy needs. And you’ll need to provide at least one on-site car parking space for the inhabitants.

Your ancillary dwelling will also need to comply with the Building Code of Australia. That means it will need a proper kitchen, bathroom, and laundry; minimum ceiling heights; connections to electricity and running water; sewer or septic connection; structural engineering certification; and compliance with energy efficiency standards. If you live in a bushfire‑prone area, you’ll also need a BAL report.

Who can Live There?

These days, anyone. There are no restrictions on leasing your ancillary dwelling out on the open market. In fact someone seeking an affordable home might appreciate you doing so.

But it is important to remember that your ancillary dwelling is not on its own title and you’ll need to consider whether you need sub meters for power, water and gas; what additional fencing you may need; and the  lease will also need to stipulate exactly what part of your land your tenant has access to. Having a granny flat will also affect your GRV valuation and may increase your rates, especially if it is leased out.

If you are wanting to help out family, you should also take a look at our information on Multi-generational living.

Do I need approval?

Yes. Before you start building your ancillary dwelling, you generally need a number of approvals from the Shire:

Environmental Approval

If your block is mapped as a Local Natural Area, or if there’s native vegetation or a watercourse that could be impacted, it’s a good idea to organise an Environmental Asset Inspection. The Shire can arrange this for you—just get in touch. Ideally, you’d line up this inspection at the same time the Bushfire Practitioner visits to do the BAL assessment.

To minimise environmental impacts, ancillary dwellings should go in areas that are already cleared or degraded, and preferably close to the main house. That way, the asset protection zones overlap, and you don’t need to remove as much vegetation

Health Approval

If your property uses septic tanks, you need approval from Environmental Health to ensure your system can handle the extra bathroom or kitchen.

Planning (Development) Approval

You likely need this if you live in a bushfire-prone area, on a heritage property, or if your design doesn't per meet the deemed-to-comply residential design codes (setbacks, size, etc.). You can find out more about getting a Planning Approval here

Prior to applying for planning approval, Shire Officers are able to undertake a preliminary assessment of your proposal, which would allow for any potential issues to be identified early on. For this to be done, a site plan, elevations and floor plan showing the proposed development, and a $73 fee, will need to be provided to the Shire’s Planning department.

Building Permit

The final step is to seek a Building Permit. This is required for all new structures or major internal changes and is about ensuring that the new home you build is structurally sound and meets all of the requirements that will make it a safe and comfortable place to live.

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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