Granny Flat / Secondary Dwelling Perth: Cost, Rules and Rental Potential

A granny flat — officially an "ancillary dwelling" under WA's planning rules — is one of the more efficient ways to add value or income to a Perth property, whether the plan is to house a parent, generate rental income, or just get a proper self-contained space off the back of the main house. It's also one of the more misunderstood, because the planning rules and the building rules are two separate things, and both matter.

WA's rules on ancillary dwellings

Under the Residential Design Codes of WA, an ancillary dwelling can be built on the same lot as an existing single house, provided it stays under the deemed-to-comply threshold — currently a maximum internal floor area of 70m², measured within the finished wall surfaces and excluding any garage. Stay within that cap and meet the standard setback requirements, and the ancillary dwelling generally doesn't need separate planning approval before you apply for a building permit; go over it, or fall outside a setback, and it gets referred for discretionary assessment by your local government, which adds time.

The other rules worth knowing before you plan a layout:

• Only one ancillary dwelling is permitted per lot, and it must sit on the same title as the main house — it can't be subdivided off or sold separately
• Since April 2024 there's no minimum lot size requirement for an ancillary dwelling in most zones, which has opened up smaller Perth Metro blocks that previously couldn't fit one
• An ancillary dwelling can be rented out independently of the main house — it doesn't have to be occupied by a family member, though many are
• Local government schemes can still add their own conditions on top of the R-Codes, particularly around parking and streetscape, so check with your council before finalising a design

Cost ranges: basic vs high-spec

A granny flat costs more per square metre than the main house it sits behind, because a kitchen, bathroom, laundry and separate services connection all get squeezed into a much smaller footprint — the fixed-cost items don't shrink just because the building does. Against the per-m² benchmark for a standard Perth new build (see our cost to build a house in Perth guide), expect a basic, standard-finish ancillary dwelling near the 60–70m² cap to land toward the upper end of that per-m² range, and a high-spec version — raked ceilings, stone benchtops, a full-height glazed living area — to push meaningfully past it. As with any build, the number that matters is the one after site costs, service connections and council fees are added, not the headline construction figure alone.

With transparent, itemized pricing, that number is disclosed as it's invoiced rather than bundled into a single fixed figure — which matters more on a smaller build like this, where a fixed price has less scope built in to absorb a site surprise.

Rental yield and other use cases

The most common reasons Perth homeowners build an ancillary dwelling are, in order: housing an ageing parent close by without merging households, generating rental income from a self-contained unit while keeping the main house, and simply wanting a separated home office, studio or space for an adult child. If rental income is the goal, a self-contained one or two-bedroom unit in established Perth Metro suburbs typically rents well against its build cost, because tenant demand for smaller, private, self-contained dwellings has stayed strong — though actual yield depends heavily on suburb, block size and finish level, and it's worth getting a local property manager's opinion on achievable rent before committing to a spec.

How this fits a transparently priced build

Because an ancillary dwelling is a smaller, more contained project than a full house, it's a genuinely good fit for a transparently priced residential build — the disclosed management fee stays proportionate to the smaller job, and you see exactly what the kitchen, bathroom and services connections actually cost rather than trusting a bundled fixed price on a project too small to have much margin buffer built in. If the site has any slope, retaining or access complications — common on Perth Hills blocks around Mundaring, Kalamunda, Roleystone and Serpentine-Jarrahdale in particular — it's worth getting earthworks scoped early, since those costs can shift the economics of a granny flat more than they would on a full-sized new build. If your block is large enough to subdivide rather than just add an ancillary dwelling, it's worth checking our free subdivision calculator and comparing the numbers against a duplex build before committing to a granny flat.