Note: This article is general planning information only. It is not legal, building, tax or financial advice. Always check your property, council/certifier pathway and numbers before spending heavily on designs, deposits or approvals.

A granny flat can be a useful investment, but the numbers need to be checked before the design takes over the decision. The advertised building price is only one part of the project.

For Queensland homeowners, the better question is not just “How much is the granny flat?” It is “What will the whole project cost, and what is the likely return after realistic expenses?”

Start with the full project cost

• Granny flat or modular home supply cost
• Plans, drafting, engineering and certification
• Site works, footings, retaining, drainage and access
• Water, sewer/septic, stormwater, power and connection costs
• Delivery, crane or truck access costs
• Decks, stairs, awnings, parking and landscaping
• Contingency for surprises

Then compare rental income realistically

Use a conservative rent estimate, allow for vacancy, allow for maintenance and check whether the finance cost changes the final cashflow. A project can look strong before these items are added, then become much tighter when the full picture is included.

Use ROI before choosing the model

It is tempting to choose a favourite design first. A smarter path is to check the site, likely budget and rental target first, then compare models that fit the numbers.