The gap between a renovation quote and the final invoice is rarely caused by the work quoted. It is caused by the work that was never in the quote in the first place.
Here is what commonly sits outside a renovation quote, and what to ask about each.
Services upgrades. An older Redlands home often needs the switchboard upgraded, the electrical brought to current standard, or the plumbing replaced where it is disturbed. Some quotes include this. Many assume the existing services are adequate.
Drainage and stormwater. Once you change a roof line or add a footprint, stormwater has to go somewhere compliant. On sloping bay side blocks this can be significant and it is frequently excluded.
Site works and access. How the machinery gets in, what has to be removed, whether the ground is what everyone assumed. On a tight site this is real money.
Asbestos and hazardous material removal. Common in homes of a certain age in this region, and almost never included as a priced item, because nobody knows it is there until a wall opens.
Termite and structural repair. Found during demolition, not before.
Council and certifier fees, and the reports behind them. Sometimes in, sometimes out, rarely obvious.
Temporary accommodation. Not a building cost, but a real one. If the house is uninhabitable for part of the build, that is a number people forget.
Making good beyond the work area. The renovation stops at a line. The paint, flooring and finishes on the other side of that line may not match, and joining old to new is its own decision.
How to check
Ask for the exclusions in writing. Not a conversation, a list.
Then ask which of those exclusions the builder thinks is most likely to apply to your house specifically, and what it would cost if it did. A builder who has walked your house can answer that. A builder who priced from drawings cannot.
Why we investigate first
Most of the items above can be found before pricing rather than during construction. That is the entire purpose of investigating the house properly at the feasibility stage. It costs money up front and it converts unknowns into priced items, which is the difference between a fixed price and a hopeful one.
You will never get that number to zero. An existing house always holds something. But you can get it small enough that it is a manageable contingency rather than the thing that derails your project.
