A granny flat is a self-contained second dwelling on an existing block, called an ancillary dwelling in WA and capped at 70m2 of rentable area. On The Property Plug panel they start at $217,500 for the build and complete in around 8 months. Because the land is already owned, the rent it earns lands almost entirely on the build cost, which is what makes the yield maths work.
The reason investors keep returning to granny flats is simple: you are buying a second income without buying a second block. The land is sunk cost. Every dollar of rent the flat earns is measured only against the build, which is why a well-placed granny flat can lift the yield of an existing property more cleanly than almost any other move.
The WA 70m2 rentable rule
In WA a granny flat is formally an ancillary dwelling. As at June 2026 it can generally be up to 70m2 of internal floor area, sit on the same lot as a main house, and crucially be rented to anyone, not only family. That last point is the engine of the investment case. A compliant flat can carry its own tenant and its own lease, separate from the front house.
Eligibility depends on your lot size, local council requirements and setbacks, so the 70m2 figure is the ceiling, not a guarantee for every block. The Property Plug panel granny flats are designed to sit within the rule, from compact one-bedroom dwellings to larger layouts at the limit.
A worked yield example
Take a $217,500 granny flat let at $480 a week. That is $24,960 a year in rent against the build cost, a gross yield of about 11.5 per cent before siteworks and holding costs. Because you already own the land, that figure is measured on the build alone, which is why granny flat yields read higher than a standalone house.
Walk the numbers through. The table holds the build cost fixed and shows what different weekly rents do to the gross yield on the build. These figures are illustrative and exclude siteworks, land, finance and holding costs, which a full cashflow would include.
| Weekly rent | Annual rent | Build cost | Gross yield on build |
|---|---|---|---|
| $420 | $21,840 | $217,500 | 10.0% |
| $480 | $24,960 | $217,500 | 11.5% |
| $540 | $28,080 | $217,500 | 12.9% |
The honest caveat: gross yield is not cashflow. Subtract siteworks, finance interest, rates, insurance and management, and the net figure is lower. But because the land is already yours, even the net return on a granny flat usually beats adding a second freestanding property, where you must buy land all over again.
Why the dual-income model works
The case rests on four things, in order of weight.
- The land is already paid for. You own it, so the rent is earned against the build alone.
- Two incomes, one title. A compliant flat is leased separately, so the block carries two rents.
- New-build depreciation. A brand-new dwelling and its fittings depreciate, improving the after-tax position for investors.
- Lower entry than a second house. A $217,500 build is a fraction of buying another freestanding property.
To price a flat for your suburb, pair this with our siteworks explainer and the cost to build by suburb data, then browse the granny flat range and your investor options.
Yields are illustrative, use a real granny flat base build price of $217,500 as at June 2026, and exclude siteworks, land, finance and holding costs. Confirm lot eligibility, council rules and a real rent appraisal for your block before relying on any figure.