Granny flats have become one of the smartest property moves you can make in Perth. Whether you’re looking to house a family member, create a rental income stream, or build a home office that’s actually separate from the main house, a well-built granny flat adds real value to your property.
And if you’re building one in WA, there’s a strong case for using steel framing over timber. Here’s everything you need to know.
The Granny Flat Boom in WA
Western Australia’s granny flat regulations changed significantly in 2024. Under the updated rules, granny flats (or “ancillary dwellings”) no longer require planning approval from your local council in most cases. You still need a building permit, and the structure needs to comply with the Building Code of Australia and relevant Australian Standards, but the removal of the planning layer has made the process faster and more accessible for homeowners across Perth.
This change has led to a sharp increase in enquiries from owner-builders, property investors, and families looking to make better use of their blocks.
Why Steel Frames Make Sense for Granny Flats
There are several reasons steel framing is particularly well suited to granny flat construction in Perth.
Termite resistance. Termite damage is the single biggest structural risk to residential buildings in WA. Steel frames are completely immune to termite and borer attack, which means no ongoing chemical treatments, no inspections for hidden damage, and no risk of a nasty surprise ten years down the track.
Speed of construction. Steel frames are precision-manufactured off site using computer-aided design software. Every component arrives cut to spec, pre-punched for services, and ready to assemble. For a typical granny flat, the frame can go up in a day or two, compared to several days for a timber equivalent. When you’re building in your own backyard and want to minimise disruption, that speed matters.
Dimensional accuracy. Because steel frames are CNC-manufactured, they’re accurate to the millimetre. That means straighter walls, tighter joins, and fewer headaches for the trades that follow: plasterers, cabinetmakers, and tilers all benefit from a frame that’s dead straight and square.
Design flexibility. Steel can span longer distances than timber without needing internal supports. This is particularly useful in granny flat design, where open-plan living spaces make a small footprint feel much larger. If you want a 6-metre clear span across your living area without a post in the middle, steel is the way to do it.
Durability in WA conditions. Steel won’t warp, twist, shrink, or swell with changes in temperature and humidity. In Perth, where summer days regularly hit 40 degrees and coastal areas deal with salt-laden air, that stability is a genuine advantage over timber.
What Size Granny Flat Can You Build?
Under WA regulations, an ancillary dwelling can be up to 70 square metres of internal floor area on most residential lots. Some local government areas have additional requirements around setbacks, parking, and lot size, so it’s worth checking with your council or a building surveyor early in the process.
Within that 70 square metre limit, you have plenty of room to work with. A well-designed one-bedroom granny flat with an open-plan kitchen and living area, a full bathroom, and a covered outdoor area can be incredibly comfortable and functional.
Two-bedroom layouts are also achievable within the limit, though they require more careful planning to make sure the living spaces don’t feel cramped.
The Build Process with Steel Frames Trusses WA
When you come to us with a granny flat project, the process typically follows five stages:
- Design and consultation. You share your plans, sketches, or ideas with our team. If you already have certified drawings, we can work directly from those. If you’re still at the concept stage, we can help shape the design to suit your block, your budget, and the relevant building codes.
- Engineering and detailing. Our in-house team uses 3D modelling and design software to engineer the frame, trusses, and floor system. Every component is designed to meet Australian Standards, including the appropriate wind rating for your location.
- Manufacturing. Once the design is approved, we manufacture the frame in our facility using precision CNC equipment. Every piece is labelled, bundled, and prepared for transport.
- Delivery. We deliver the complete frame package to your site, ready for installation.
- Installation. Our experienced installation team can erect the frame on site, or if you’re an owner-builder or working with your own crew, we provide detailed installation documentation to guide the process.
How Much Does a Steel Frame Granny Flat Cost?
The frame and truss package for a granny flat can range in price depending on the size, design complexity, and wind rating. This is a fraction of the total build cost, which will also include the slab, cladding, roofing, plumbing, electrical, internal fit-out, and site works.
Total turnkey costs for a finished granny flat in Perth currently range from about $120,000 to $200,000 or more, depending on the specification and finishes you choose. A steel frame sits at the more cost-effective end of the materials spectrum when you factor in speed of construction, reduced waste, and long-term maintenance savings.
Ready to Get Started?
If you’re thinking about building a steel frame granny flat in Perth, we’d love to help. Whether you need just the frame kit or the full package from design through to installation, our team has the experience to deliver a quality result.
