1 Bedroom Granny Flats WA Price List 2026: Real Costs for a Perth Ancillary Dwelling
Summary
The short version. A one bedroom ancillary dwelling in metro Perth lands between $120,000 and $185,000 all up in 2026, for something in the 45 to 60 square metre range. That works out to roughly $2,200 to $3,400 per square metre once you include site works, connections, and approvals. The advertised numbers you see on billboards and Facebook are usually the base build only, and the gap between that figure and your final bank statement is commonly $35,000 to $60,000. Regional WA adds 8 to 15 per cent, and further north than Geraldton it climbs faster than that. If you are running the job yourself, the two costs that catch people are the sewer or septic decision and the second power connection.
Table of Contents
One of our Baldivis clients told us last year that he had been quoted $99,000 for a one bedroom granny flat. He had not signed anything, so he rang three separate suppliers to sanity check the price. All three came back with a similar base figure.
What none of those quotes included was the sand pad, second crossover, sewer junction extension or certificate of design compliance. By the time he had a realistic project cost in front of him, the figure had climbed to $154,000 and four months had passed.
That gap is the real story of granny flat pricing in Western Australia. In WA these builds are classed as ancillary dwellings, and under the Residential Design Codes they can be up to 70 square metres in floor area, with one permitted per lot alongside the existing home. Since 2013, they can also be rented to anyone rather than being restricted to family members. A typical one bedroom design sits somewhere between 45 and 60 square metres.
For metro Perth in 2026, a realistic budget is around $120,000 to $185,000. If the quote comes in well below that, look closely at what has been left out.
What a one bedroom ancillary dwelling actually costs in WA
Perth is the cheapest capital in the country to build a small dwelling in, and it is not close. Sandy soils mean most metro blocks are a Class A or Class S site, so footings are simple. Trades are more available than they were in 2022. Transport distances are short if you are inside the Mitchell and Kwinana corridors.
The honest 2026 range for a completed, connected, occupiable one bedroom ancillary dwelling in metro Perth is $2,200 to $3,400 per square metre. At the bottom sits a 45 square metre build with a basic kitchen, vinyl plank throughout, and a single split system. At the top sits a 60 square metre build with stone benchtops, a proper laundry, decent glazing, and a small alfresco under the main roof.
So: $105,000 for a lean 48 square metre unit on a flat, sewered, easy block. $185,000 for a well finished 60 square metre unit with retaining, a new crossover, and a septic system. Both are real. The difference is almost never the building itself.
From first sketch to keys: the price list and the calendar
Owner-builders underestimate the front half of this process almost universally. The construction takes four or five months. Getting to the point where construction can legally start takes another three or four if you are learning as you go.
| Stage | Realistic duration | Cost range (metro Perth, 2026) |
|---|---|---|
| Site measure, feasibility, R-Codes check | 1–2 weeks | $0–$1,200 |
| Working drawings and documentation | 3–6 weeks | $3,500–$7,500 |
| NatHERS energy assessment (7 star) | 1–2 weeks | $450–$900 |
| BAL bushfire assessment (if mapped) | 1–3 weeks | $400–$800 |
| Engineering: footings, tie-down, retaining | 2–3 weeks | $1,800–$4,500 |
| Development approval (only if non-compliant) | 60–90 days | $150–$1,600 |
| Owner-builder approval, Building and Energy | 4–8 weeks | $200–$450 |
| Certificate of design compliance (BA3) | 1–2 weeks | $700–$1,600 |
| Building permit, certified (BA1) | 10 business days | 0.19% of value, min $110 |
| Building Services Levy | with permit | 0.137% of value, min $61.65 |
| Construction Training Fund levy | before permit | 0.2% of value over $20,000 |
| Site works, pad, and slab | 2–4 weeks | $9,000–$28,000 |
| Frame, roof, lock-up | 6–9 weeks | $55,000–$85,000 |
| Fit-out, services, practical completion | 6–10 weeks | $38,000–$62,000 |
Add it up and the paperwork alone is $8,000 to $18,000 before a single truck arrives. That is not a rort. It is the cost of a permit that holds up when you go to insure, rent, or sell the thing.
Stage by stage, where every dollar goes
The order you spend money in matters more than the total, because each stage locks in the one after it. Get the documentation wrong and you pay for it three times over in variations.
Drawings, consultants, and the R-Codes test
Every ancillary dwelling in WA gets tested against the deemed-to-comply provisions of the R-Codes before anything else. Floor area under 70 square metres, setbacks, open space, and the car bay requirement for the R-Code applying to your lot. Meet all of them and most local governments will take a building permit application without a separate development approval, which saves you two to three months and $150 to $1,600.
Working drawings for a one bedroom ancillary dwelling run $3,500 to $7,500 depending on how much of the site is complicated. Add the NatHERS assessment at $450 to $900, since WA adopted the NCC 2022 energy provisions on 1 May 2025 and 7 stars is now the floor, not the aspiration. Engineering is $1,800 to $4,500, and it climbs sharply if you need retaining over 500mm.
Approvals, levies, and owner-builder registration
If you are not engaging a registered builder and the work is valued over $20,000, you need owner-builder approval from Building and Energy before the permit application goes in. Allow four to eight weeks and $200 to $450. You cannot have held one in the previous six years, and you must own the land. Plenty of people find that out in week nine of a twelve week plan.
The permit itself is 0.19 per cent of the estimated value for a certified application, plus the Building Services Levy at 0.137 per cent, plus the Construction Training Fund levy at 0.2 per cent. On a $150,000 build that is roughly $735 in levies and fees. Certified applications are decided in ten business days. Uncertified take twenty five. Get a private building surveyor to issue the certificate of design compliance and take the faster path.
Site works, the pad, and the slab
This is where Perth blocks diverge. A flat, sandy, sewered lot with clear side access might cost $9,000 to $14,000 for pad, slab, and setout. A sloping Hills block in Kalamunda or Mundaring with limestone retaining, a crane hire for materials, and rock in the trenches will pass $28,000 without trying.
Order a site classification and a dial-before-you-dig before you accept any slab price. We have watched two owner-builders discover a sewer main running diagonally under the proposed slab, and neither of them had budgeted for the Water Corporation build-over application.
The build itself, then the connections
Frame, roof, and lock-up is the single biggest cheque at $55,000 to $85,000, and it is the most predictable one. Fit-out is $38,000 to $62,000 and it is where owner-builders overspend, because tiles and tapware feel like the fun part after nine months of forms.
Connections are the quiet killer. A separate power supply and meter through Western Power runs $1,500 to $4,000. Sewer junction extension is $3,500 to $9,000. If you are unsewered, an approved on-site wastewater system is $8,000 to $15,000 and needs Department of Health approval through your local government. A second crossover, if the R-Code requires the extra bay, is $3,500 to $9,000.
Before you commit a dollar
Run your numbers through our owner-builder budget checklist and check whether your build needs a development approval or just a building permit with our permit checker. Both are free. Both will take you fifteen minutes and save you the four months our Baldivis caller lost.
The line items that never make it onto the quote
There is a pricing habit in this industry that we find hard to defend. A supplier advertises “$99,000 complete” and technically means it, because “complete” refers to the building, delivered to a level pad, connected to services that already exist, on a lot that needs no approval beyond a permit. It is the same trick as an appliance advertised at $1,499 where delivery, installation, and removal of the old unit are three separate line items you discover at checkout. The headline is real. The transaction is not.
Here is what routinely sits outside the advertised figure: site works and pad preparation, retaining, all approval fees and levies, energy and bushfire assessments, engineering, sewer or septic, power connection and metering, stormwater soakwells, crossovers, fencing, paving, clothesline, letterbox, and landscaping. That list is commonly $35,000 to $60,000 in metro Perth. On a difficult block it is more.
We are a drafting firm, so we should be plain about where we sit in this. We draw the plans, we don’t sell buildings. We have no supplier arrangement, no kit to move, and no reason to talk you into 60 square metres when 48 does the job. That also means we will tell you when your block is a bad candidate, which a company holding stock in a Wangara yard has no commercial reason to do.
The test we give every owner-builder is simple. Ask any supplier to put their exclusions in writing before you pay a deposit. A straight operator will send you a list. Everyone else will send you a phone call.
What Draftee does
We document residential projects for owner-builders across NSW, VIC, QLD, TAS, and WA. Working drawings, R-Codes and planning checks, consultant coordination, and the documentation set your building surveyor needs to issue a certificate of design compliance. We hold no stock, take no supplier commissions, and have no product to move. Our fee is the same whether your ancillary dwelling ends up at 45 square metres or 68.
How to hold the price down without wrecking the build
The cheapest savings in a one bedroom ancillary dwelling are all made on paper, before anyone breaks ground. The expensive savings are the ones you try to make halfway through.
Design to the deemed-to-comply provisions rather than seeking variations. A development approval you avoid is two to three months and up to $1,600 you keep. Keep the wet areas of the ancillary dwelling on the same side of the block as the existing house, since plumbing runs of fifteen metres versus five metres is a $3,000 difference in trenching and pipework alone. Keep the roof form simple: a skillion or a basic gable costs less to frame, less to flash, and leaks less. Stay under 60 square metres, because the last ten square metres of a 70 square metre unit buy you very little rentable value and cost you $22,000 to $34,000.
Then spend the money where it cannot be retrofitted. Insulation, glazing, waterproofing, and the slab. Vinyl plank can be replaced in year eight for $4,000. A badly detailed shower cannot.
Nine out of ten budget overruns we see in WA trace back to a decision made in the first three weeks. Not the last three months.
Ready to draw?
Send us the address, a rough idea of what you want, and the R-Code if you know it. We will tell you what the block can actually take and what the documentation will cost, in writing, before you spend anything. Tell us about your block
Continue Your Research
Knock Down Rebuild Cost 2026: What the Whole Job Actually Costs
Sloping Block House Design: Cost, Approvals, and What the Fall Decides
The Importance of Professional Land Surveys: Why the Fence Isn’t the Boundary
Draftee provides architectural drafting and documentation services. We do not provide legal, planning, structural, or construction advice, and nothing in this article is a quotation. Costs and fees quoted are indicative for metro Perth in 2026 and are drawn from quotes and projects we have reviewed. Local government fee schedules, the Building Services Levy, and Construction Training Fund levy rates change, so confirm current figures with your local government and Building and Energy before you budget. The R-Codes provisions cited are from State Planning Policy 7.3 Volume 1 and may be modified by a local planning policy or scheme in your area. WA adopted the NCC 2022 energy efficiency provisions on 1 May 2025, and NCC 2025 transitional arrangements may affect projects documented late in 2026. Verify all regulatory detail with your local government and your registered building surveyor before acting on it.
Last updated and changelog
Last updated: 6 September 2026
Changelog:
– 6 September 2026: Initial publication
