Add a contingency percentage and any costs outside the contract to see the budget you should actually plan for.
A renovation budget that equals the contract price is a budget that will be overspent. Old houses hide rotten bearers, out-of-level floors, non-compliant wiring and asbestos; owners change their minds; and prime cost and provisional sums are often set low. This calculator adds a contingency on top of the price and then adds costs that sit outside the builder's contract.
How to use it. Enter the contract price or your estimate including GST. Choose a contingency percentage: an illustrative rule of thumb is around 10% for a straightforward job in a newer home and 15% to 20% for older homes, structural work or incomplete plans. Use the fixed buffer for a specific risk you can price, and the last field for council or certifier fees, temporary rent, storage or removalists that the builder will not invoice.
Formula. Contingency = price x percentage / 100 + fixed buffer. Total budget = price + contingency + costs outside the contract.
Worked example (illustrative). A $60,000 bathroom and laundry contract with a 10% contingency adds $6,000. With $2,500 of costs outside the contract the total budget is $60,000 + $6,000 + $2,500 = $68,500.
Limits. Contingency is money you hope not to spend; it is not a target for the builder. Keep it out of sight of variation negotiations, and remember that variations still need to be agreed in writing under state contract laws. The percentages are illustrative, not an industry standard.
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