Why do renovations go over budget?

Three things do most of the damage: quotes given without drawings or a written spec, so builders price the unknowns; hidden problems that only appear once walls and floors are open, like asbestos, rotten stumps, old wiring and corroded plumbing; and variations, the small changes made mid-build that each add cost. The average renovation runs 20 to 40 percent over its starting number.

How much contingency should I hold for a renovation?

A well-specified job on a modern house needs 10 to 15 percent. A pre-1980 house, or any job without drawings and a spec, needs 20 percent. Both together, or a weatherboard on stumps, justify 25 percent. The calculator recommends a figure from your own inputs rather than a flat rule.

Why is labour such a large share of a renovation budget?

On a builder-managed renovation roughly two-thirds of the spend is labour and coordination, not materials. Every fixture you buy has to be demolished around, framed for, plumbed, wired, tiled, painted and cleaned up after. Managing trades yourself lowers the share a little; doing the work yourself lowers it a lot.

Is the calculator a quote?

No. It is an indicative reality check calibrated to Australian 2026 costs, meant to show what typically lands on top of a quote or budget and why. Always get written quotes against a specification, and use the result to decide how much buffer to hold.

Renovation Blowout Calculator: What Your Quote Will Really Cost

How this works

Most calculators guess what a renovation costs. This one starts from a number you already have, a quote or a budget, and works out what typically lands on top of it. That is the figure that catches people out: the average Australian renovation finishes 20 to 40 percent over where it started, and almost none of that shows up in the first quote.

The result is built from four things. Hidden costs are the problems that only appear once walls and floors are open, and the calculator weighs each one by how likely it is for your house: asbestos in a 1950s to 1980s home, rotten stumps under a weatherboard, wiring and galvanised pipes in anything pre-1980. Variations are the small changes every build accumulates, typically 10 to 15 percent of the works, more in a pre-war house. Uncertainty is how much the number can drift before anyone has priced it properly, which is why drawings and a written spec narrow it so much. And if you entered your own estimate but plan to use a builder, their margin and coordination gets added, because a quote includes it and an estimate usually does not.

The contingency recommendation comes from those same inputs rather than a flat rule, and the split between labour and materials reflects who is running the job. On a builder-managed renovation roughly two-thirds of the money goes to people, not products, which surprises almost everyone the first time they see it.

The ranges are indicative and dated September 2026, drawn from what Australian renovators actually paid in 2025 and 2026 rather than from a pricing database. They will drift, and no calculator replaces a written quote against a specification. Use the result to decide how much buffer to hold, then track the real numbers as the build happens. If you are still working out the total you can afford, the step-by-step budgeting guide covers that first.