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Granny flat costs in Western Australia

From Modular Living Australia — the independent knowledge base Last updated: 8 Oct 2026

Published Perth figures put a turnkey one-bedroom granny flat at about A$105,000–A$152,000 and a two-bedroom of about 60 m² at A$133,000–A$200,000, with site works and connections adding A$14,000–A$38,000.^[1] Most 70 m² builds in Western Australia are put at A$150,000–A$190,000.^[2] These are commercial guidance, not independent survey data.

Western Australia has the largest suburban blocks in the country and, since 2024, some of its least restrictive rules for secondary dwellings. The published cost figures reflect that: the Perth ranges sit noticeably below the eastern capitals for comparable dwellings. This guide sets out what those figures say, how the 70 square metre cap is measured — which is not how caps are measured elsewhere — and which costs sit outside the advertised number.

What the published sources say §

Construction cost in the Perth metropolitan area is put at approximately A$2,200 to A$3,250 per square metre^[3] — a band that sits below the A$2,500 to A$4,000 published for Brisbane and materially below the Melbourne figures. Whether that reflects a genuinely cheaper market or simply different inclusions in the underlying quotes is not something the published sources establish.

The 70 m² cap is measured differently §

This is the detail that most affects a Western Australian budget and is most often missed.

The 70 square metre limit for an ancillary dwelling applies to enclosed internal living area. Verandahs, patios and carports are not counted toward it, subject to the local government's site coverage rules^[2].

That is a meaningfully more generous position than a cap applied to total area, and it changes what the money buys. A 70 m² internal dwelling with a covered outdoor area and a carport is a substantially larger building than 70 m² implies, and outdoor structures are among the cheapest square metres in any build. Where a budget is tight, moving amenity outdoors is the most efficient move available in WA and is not available in the same way in states that measure differently.

The comparison consequence also matters: a WA 70 m² figure and an eastern-states 60 m² figure are not measuring the same thing, and per-square-metre rates should not be carried across borders without checking the basis.

What the 2024 reforms changed for cost §

The 2024 Western Australian planning reforms removed the previous 350 m² minimum lot size for an ancillary dwelling, and a design that is deemed to comply with the Residential Design Codes does not require a planning permit — only a building permit^[2][6]. Ancillary dwellings can be rented to tenants who are not family members^[6].

For budgeting, the relevant effect is the removal of a planning application from most projects: no application fee, no advertising, no assessment period. The design work needed to stay inside deemed-to-comply is not free, but it is cheaper than a discretionary approval. Whether a specific lot qualifies should be confirmed against Granny flat rules in Western Australia and with the local government.

Why the ranges diverge §

The A$76,000 figure and the A$404,000 figure describe different transactions, not different opinions about the same one.

At the low end sits a prefabricated or modular unit quoted as supplied^[1] — a building delivered to the block, with the slab, connections, approvals and site works still to be paid for. At the top sit custom and two-storey builds^[1] and large designer dwellings with carports, garages and verandahs^[4], where the additional structures are outside the 70 m² calculation but very much inside the invoice.

Between them, the practical spread for a mainstream Perth project is roughly A$105,000 to A$200,000 turnkey^[1], with one builder placing a basic one-bedroom at around A$160,000^[5] and another quoting an investor-specification one-bedroom from A$130,000^[4]. Two builders differing by A$30,000 on nominally the same product is a specification difference, and the inclusion lists are the only way to see it.

Site works are the underestimated item §

Site works and service connections are put at A$14,000 to A$38,000^[1] — a A$24,000 spread on a project whose whole build might be A$150,000.

This is the category that turns a competitive quote into an average one, and it is site-specific rather than negotiable: distance to the sewer main, fall across the block, sand versus clay, access for machinery, whether the existing switchboard can carry a second dwelling. A quote that does not state its site works assumption has not priced the project.

One builder recommends a contingency of 5 to 10 per cent on top of the estimate, particularly on older or unusual blocks^[5]. That is lower than the 15 to 20 per cent recommended for Melbourne, which is consistent with Perth's flatter blocks and more predictable sandy soils, but it is a builder's figure rather than an independent one.

Programme §

Modular and shipping-container builds are stated to be on site within 6 to 8 weeks once construction begins^[6]. Programme is a cost where the project is financed or where the owner is paying rent elsewhere, and it is one of the few areas where WA's prefab-heavy market has a measurable advantage over conventional construction.

The Perth figures above, including the separately stated site works allowance, are in the cost range finder alongside every other state.

What is not covered here §

No independent Australian survey of completed Western Australian ancillary dwelling costs has been located for this guide. Regional WA costs, which differ substantially from Perth metropolitan rates because of freight and trade availability, are not covered by the sources compiled here. Local government fees vary and are not covered. Rental yield and property value uplift figures circulate widely in this market and are promotional; they are not reproduced. Tax treatment of a rented ancillary dwelling is not covered here.

References §