What our analysis shows
Analysis by Granny Flat Numbers, data as of 23 October 2026
- A standard 60 m² two-bedroom flat comes to $198,500 turnkey in Perth ($174,500–$222,000) and $169,000 in regional WA. The full 70 m² that WA allows costs $225,500.
- 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 with no council contribution, against $10,000 in NSW once the council contribution is added.
Weekly rent needed for a 10-year payback in WA
- 45 m², 1 bed, Perth$515
- 60 m², 2 bed, regional$540
- 60 m², 2 bed, Perth$630
- 70 m², 2 bed, Perth$710
- 60 m², premium finish, Perth$780
After agent fees, insurance, maintenance and tax at 30%, no loan. WA does not publish bond medians by area, so we show the rent you need rather than a payback.
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 BASIX/energy report | $8,000 |
| Approval: Building permit (no development approval) | $4,500 |
| Total | $198,500 |
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 | $119,500 | $138,500 | $170,000 | $120,000 |
| 1 bed, 45 m² | $138,000 | $161,000 | $199,000 | $138,500 |
| 2 bed, 60 m² | $168,500 | $198,500 | $247,500 | $169,000 |
| 2 bed, 70 m² | $190,500 | $225,500 | $283,000 | $191,000 |
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 | $530 | $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, not an apartment. In Sydney, where we could measure it, detached one-bedroom dwellings rented for about a quarter less than apartments. In Hobart and Launceston there was no such gap. Then read across to see how long your flat takes to pay back.
60 or 70 m²
The extra 10 m² costs $27,000 and needs $80 a week more rent to keep a 10-year payback. Tenants rarely pay that for a bigger living room alone. It is worth it when it buys a better second bedroom, a separate laundry, or space an older parent needs for mobility aids.
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. Some councils may require an extra car bay unless the lot is close to frequent public transport. If any standard is not met, you apply for development approval against the design principles, which our model prices at $4,500 more.
WA-only costs
- Home indemnity insurance. Required from the builder on residential work over $20,000.
- Building Services Levy and Construction Training Fund levy. Both are charged on the value of the work with the building permit.
- Water Corporation. A separate meter or sewer junction needs Water Corporation approval and fees.
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 $189,500 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.
Questions people ask
How much does a granny flat cost in Perth?
About $198,500 turnkey for a standard 60 m² two-bedroom flat, including connections and the building permit. Budget: $168,500. A 70 m² flat: $225,500.
Do I need council approval for a granny flat in WA?
Not development approval, if the flat is up to 70 m² and meets the R-Codes deemed-to-comply standards (setbacks, open space, height). You always need a building permit.
Is there a minimum lot size for a granny flat in WA?
Not since 10 April 2024. The old 450 m² minimum (350 m² under some local schemes) was removed from the R-Codes.
How big can a granny flat be in WA?
70 m² of internal floor area under the deemed-to-comply standard. Larger flats can be approved through development approval against the design principles, and some councils' policies allow more.