A granny flat sounds simple until you start reading the actual rules — floor area limits, design matching, deemed-to-comply pathways, building permits. Most of it comes down to three things: how big it can be and what it has to look like, how it gets approved, and what you can and can’t do with it once it’s built. Here’s what actually applies in WA, in plain English.

Size and Design Matching

WA’s general floor area cap for an ancillary dwelling sits around 70 square metres, though some councils apply their own figure, so it’s worth confirming your specific council’s limit rather than assuming 70sqm is guaranteed. Within that limit, a granny flat still has to work as a proper home — orientation, natural light and storage matter more here than in a full-sized house, since there’s less floor area to absorb a poor layout decision. On top of the size limit, the R-Codes generally expect a granny flat’s roofline, materials and colours to match the main house, since councils read the two as one property from the street. That means configuration matters as much as the floor plan: attached (sharing a wall with the main house), integrated (under the same roofline without shared internal access), or fully detached further back on the block. Each suits a different block shape and level of privacy between the two households, usually decided early, alongside the floor area check.

Deemed-to-Comply or a Full Application

Whether your granny flat needs a full development application or can go straight to a building permit depends on whether the design meets the R-Codes’ deemed-to-comply provisions. A design that ticks every box — floor area, setbacks, design matching, parking — can often skip a planning application entirely and move straight to building permit documentation, either a certified BA1 or an uncertified BA2. One that doesn’t needs a development application first, which adds time but isn’t unusual on tighter or irregularly shaped blocks. An additional car bay is often required too, depending on your block’s density coding, unless the site sits close to a train station or a well-served bus route. And if your block isn’t connected to mains sewer, add one more step: an on-site treatment system approved through the council’s Health department, alongside the usual building approval — worth flagging at the first site visit rather than discovering it partway through documentation.

What You Can (and Can’t) Do With It Later

A granny flat stays on the same title as your main house — it’s not a new lot, and it’s not a separate property you can sell on its own. What you can do is rent it out independently, whether that’s to family, a tenant, or as a home office or studio, without needing to change how the title works. If a separate, sellable title is what you’re actually after, that’s a subdivision, a different process with its own rules. Building a granny flat now doesn’t automatically rule out subdividing later, but it can change what a future subdivision looks like, so it’s worth raising at the first conversation rather than assuming the two are unrelated.