How We Calculate

Our calculator uses a component-based cost model. Instead of giving you a single vague range, we calculate each part of the project separately and show you the full breakdown.

The formula

1. Base build cost

The base build cost starts with a per-square-metre rate that varies by size. Smaller granny flats have a higher $/sqm because fixed costs (kitchen, bathroom) are spread over less area. We then adjust for build type (kit, prefab, or site-built) and finish quality (budget through luxury).

2. Site preparation

Starts at $20,000 for a flat site with standard access. Slope adds $7,500 (gentle) to $30,000 (steep). Restricted access or crane requirements add $4,000–$8,000.

3. Utility connections

Covers electricity, water, sewer, stormwater, NBN, and switchboard upgrade. Base cost is $18,000 (within 20m of existing services), adjusted by a state-specific factor based on published builder connection packages.

4. Design and approvals

Includes $8,000 base design and engineering fees, plus state-specific approval costs. NSW has the highest approval costs due to Section 7.11/7.12 council contributions ($4,000–$12,000). Victoria and WA are the most streamlined.

5. Contingency (10%)

We add 10% to the subtotal. Construction projects routinely encounter unexpected costs — soil conditions, council requirements, material delays. The industry standard recommendation is 10–15%.

Cost range

We show you a range (-10% to +15% from our estimate) to reflect the inherent variability in construction costs. The midpoint is our best estimate based on available data.