What our analysis shows
Our calculations, data as of 23 October 2026
- A standard 60 m² two-bedroom flat comes to $198,300 turnkey in Perth ($174,500–$222,000) and $168,800 in regional WA. The full 70 m² that WA allows costs $225,600.
- For a 10-year payback after tax, the Perth flat needs about $630 a week. A 45 m² one-bedroom needs $515, and the 70 m² flat $710.
- REIWA put Perth's median unit rent at $700 a week in April 2026. A granny flat letting at 80% of that, $560, would pay back the Perth build in 11.3 years; it needs close to the apartment median to reach 10.
- Approval is cheap in WA: a compliant flat needs only a building permit. Our model puts approval at $4,500 and there is no council contribution in WA.
Turnkey cost in Perth by size and finish
- 45 m², 1 bed, standard$161,000
- 60 m², 2 bed, budget$168,300
- 60 m², 2 bed, standard$198,300
- 70 m², 2 bed, standard$225,600
- 60 m², 2 bed, premium$247,400
Flat block, services within 12 m, building permit only, incl. GST.
What you pay for
| Line, 60 m² in Perth | Cost |
|---|---|
| Construction | $163,800 |
| Site preparation (flat, easy access) | $12,000 |
| Sewer, water, power and stormwater connections | $10,000 |
| Design, engineering and NatHERS energy rating | $8,000 |
| Approval: Building permit (no development approval) | $4,500 |
| Total | $198,300 |
Perth's sandy soils usually make the slab and site work cheaper than in Sydney or Brisbane, which is why many WA quotes come in under our flat-block figure. Double-brick construction, still common in Perth, costs a little more than the lightweight frames most east-coast granny flat builders use.
| Size | Budget | Standard | Premium | Standard, regional |
|---|---|---|---|---|
| Studio, 35 m² | $119,600 | $138,600 | $169,900 | $119,900 |
| 1 bed, 45 m² | $137,800 | $161,000 | $199,000 | $138,300 |
| 2 bed, 60 m² | $168,300 | $198,300 | $247,400 | $168,800 |
| 2 bed, 70 m² | $190,600 | $225,600 | $282,900 | $191,200 |
The rent each size needs
Western Australia does not publish bond medians by area, so instead of a payback we worked backwards: what weekly rent does each flat need to pay for itself in 8, 10, 12 or 15 years after tax?
| Payback target | 45 m², 1 bed | 60 m², 2 bed | 70 m², 2 bed | 60 m², regional |
|---|---|---|---|---|
| 8 years | $640 | $785 | $885 | $670 |
| 10 years | $515 | $630 | $710 | $540 |
| 12 years | $435 | $525 | $595 | $455 |
| 15 years | $350 | $425 | $480 | $365 |
To use the table, get a rental appraisal from a local agent for a detached one- or two-bedroom dwelling rather than an apartment, then read across to see how long your flat takes to pay back.
60 or 70 m²
The extra 10 m² costs $27,300 and needs $80 a week more rent to keep a 10-year payback. Tenants rarely pay that for a bigger living room. It pays its way when it makes the second bedroom usable, adds a separate laundry, or gives an older parent room for a walker or wheelchair.
Approval and what it costs
A compliant ancillary dwelling of up to 70 m² needs only a building permit, issued by council or through a private building surveyor's certificate of design compliance. It must still meet the R-Codes deemed-to-comply standards, including setbacks and open space; the 70 m² is measured as plot ratio area, to the outside of the walls, not counting verandahs, patios or carports. Check your local planning policy for any parking requirement. If any standard is not met, you apply for development approval against the design principles, which our model prices at $3,000 more, mostly drawings and a planning report rather than the council fee itself.
Checks specific to WA
Ask the Water Corporation for the sewer plan before you design: a sewer main across the backyard is the most common reason a Perth flat moves or needs pipe protection. Much of the Perth Hills and the urban fringe is mapped as bushfire prone, where a BAL assessment comes before the building permit and higher ratings change the windows, cladding and subfloor. The builder must hold home indemnity insurance for residential work over the statutory threshold, and the Building Services Levy and Construction Training Fund levy are charged with the building permit.
If you are borrowing, expect the bank's "as if complete" valuation to come in below the build cost, and many lenders will not count the flat's rent until a lease is signed. See financing a granny flat.
This week's worked example
Week of 19 October 2026. A new example, rule and owner report every Monday.
A 40 m² one-bedroom flat with a standard finish on a steep block, services 30 m from the house, in Western Australia: our model puts it at $189,600 turnkey ($167,000–$212,500), or $4,740 per m².
Rule check: Builder deposits are capped by law: 10% in NSW, 5% in Victoria and Queensland for contracts of $20,000 or more, and 6.5% in Western Australia.