Originally published in August 2012. Reviewed 7 October 2026. The historical example below is retained to explain the arithmetic; it is not a current building quote, rental appraisal or investment forecast.
A granny flat can add a separate home for family or a potential rental income stream to an existing Sydney property. For an investor, the useful question is how the complete project cost, achievable rent and ongoing expenses fit together on that particular property.
Start with the property and approval pathway
Check the proposed layout, access, privacy, drainage, services and planning constraints before relying on a budget. Complying development is available only where all applicable requirements are met; another property may need a council development application. Approval and construction timeframes depend on the proposal and its assessment.
Use our Sydney approval checklist and the current NSW Planning secondary-dwelling guidance as starting points for a site assessment.
Separate gross rent from profit
The original August 2012 example assumed a $100,000 build and rent of $400 per week. At full occupancy, $400 × 52 is $20,800 in annual gross rent. Dividing that gross rent by the assumed build cost gives 20.8%, but this is a simplified ratio using only the build cost. It is not a net return or a valuation of the whole property investment.
Those historic figures do not establish today’s rent or cost. They also do not show that the project pays for itself within a fixed number of years. Interest, vacancies, maintenance and other expenses continue after rent has been received. Gross rental receipts do not become pure profit once they equal the original building cost.
Build a property-specific budget
- Start with the proposed design and a written building scope.
- Allow separately for design and approvals, site investigations, drainage, services, access and any other work required for your block.
- Ask a local property manager for a rental appraisal based on comparable homes, the layout, privacy and available access.
- Allow for vacancy periods, management fees, maintenance, insurance, finance and applicable taxes when considering cash flow.
- Check how a lower rent, longer vacancy or higher project cost would affect the outcome.
ASIC’s Moneysmart property investment guidance explains costs and risks, including the possibility that rent will not cover expenses. Personal finance and tax decisions need advice suited to your circumstances.
Compare layouts and current pricing
Compare our granny flat floor plans for bedroom space, storage, outdoor privacy and how the home fits your site. A larger advertised area may include a patio, so compare the enclosed layout as well as the total area.
Our building price calculator and approval price calculator provide separate estimates. Selected options and property conditions affect the final written quote; calculator prices do not predict rental returns or investment performance.
We coordinate your design, approvals and building processes. Talk to our team about your property to identify the design and approval work needed before you commit to a project.
