Granny Flat Cost Calculator: Estimate Your 2026 Build Price

A granny flat cost calculator helps you organise assumptions before you have complete quotes. It cannot inspect your land, locate services or establish current supplier prices. Use TGFG’s result as an illustrative planning scenario, then replace the assumptions with evidence for your address.

We do not have completed-project data that establishes a percentage accuracy for this tool. The former claim that results were usually within 10–15% of a quote has been withdrawn. A precise-looking total is still an estimate if its inputs are allowances.

Understand the categories behind the total

The calculator groups the project into preliminaries, site costs, the build and finishes. Those categories help expose work that a headline kit or module price may exclude. They are not evidence that every project spends a particular percentage on each phase.

A complete module at the factory can still leave delivery and site installation unpaid. A materials kit leaves more construction to arrange. Compare those purchases using the dated supplier example in our cost guide.

Start with a defined project

Choose the intended build type, floor area and specification, then check the site assumptions. If you don’t know the slope, foundation requirements or available connection points, record that uncertainty beside the result. Guessing those inputs more confidently does not improve the evidence.

Check the intended area and planning route before commissioning a supplier quote. The NSW secondary dwelling guidance, for example, explains pathway conditions; it does not supply a construction price. Other states have different frameworks.

Save the assumptions with the date and plan revision. When you change the design, review the scope rather than simply increasing one total. A larger bathroom might affect foundations, services and fit-out, not just floor area.

Use arithmetic without turning it into a market claim

Hypothetical example: suppose a supplier contract is $150,000, separately quoted work is $25,000 and an unresolved allowance is $10,000. The provisional project total is $185,000 before any additional reserve you choose for identified risks. These invented amounts demonstrate addition; they are not prices for a particular size or suburb.

Next, check whether any of the separately quoted work is already included in the supplier contract. If $5,000 of electrical work appears in both, reconcile it before comparing the budget with another proposal. Conversely, a missing delivery quote should remain an open item rather than silently become zero.

Keep a column for the basis of each amount: fixed quote, provisional allowance or not yet priced. Our cost breakdown helps organise those records.

Investigate what moves the result

Change one assumption at a time to understand the tool’s sensitivity. That exercise shows how the model behaves; it does not establish the real cost of a retaining wall or sewer connection. Request the relevant site assessment and quote before relying on that line.

If the project only appears affordable when every uncertain item is at its lowest allowance, identify which quotes you need before committing. Ask the supplier what its price assumes about access, ground conditions and owner-supplied work. Read the quote comparison guide to align competing scopes.

Move from a scenario to a documented budget

Send the same brief to suitable suppliers, with the address and available survey information. Request inclusions, exclusions, expiry dates and payment stages. Keep GST treatment consistent across quotes and ask who pays for design or approval changes.

Review the resulting budget with the people responsible for design and construction. A calculator result is not a contract price, finance approval or property valuation. If borrowing, give the lender the actual project documents. Browse the supplier hub once you know which supply scope you want.

Frequently Asked Questions