Almost every homeowner who has been through a substantial project has a change order story, and almost all of them share a feature: the mechanism was never agreed beforehand.
Two kinds, and they are different
Discovered conditions. Rot behind the tiling. Knob-and-tube in the wall. Decking that failed once the old roof came off. Nobody could have priced these accurately because nobody could see them. These are the honest majority.
Homeowner changes. Deciding mid-project to move a wall, upgrade the worktop, or add a fixture. Entirely legitimate, but a decision you made rather than a condition you inherited — and it is worth being clear with yourself about which category you are in.
Agree the mechanism first
The single most useful thing you can do is settle, before work starts, how change orders will be priced and approved.
Get rates upfront for the things most likely to arise. What is the per-sheet rate for replacing rotten decking? What does a panel upgrade add? What is the hourly rate for unforeseen work, and what is the markup on materials?
With those agreed in advance, a discovery becomes arithmetic. Without them, it becomes a negotiation conducted while your roof is open and your leverage is at its lowest.
Written, always
Verbal change orders agreed on site are the source of most disputes about final invoices. Both parties remember the conversation differently, and neither is lying.
Insist that any change is documented before the work is done: what it is, why it is needed, what it costs, and what it does to the schedule. An email is sufficient. What matters is that it exists and predates the work.
Ask what it does to the timeline
Change orders affect the programme as well as the price, particularly where they trigger an additional inspection or require materials to be ordered. A change accepted for cost without considering schedule can produce a project that runs weeks longer than expected.
You can say no
Not every discovered condition must be addressed immediately. Some genuinely must — you cannot leave rotten decking under a new roof. Others are opportunities rather than necessities, and a contractor pointing out that the plumbing could be updated while the wall is open is giving you useful information, not an instruction.
Ask which category you are in. A good contractor will tell you honestly, because their reputation depends on the answer being reliable.
The pattern that should concern you
A single change order on an older home is normal. A steady stream of them on a project that was quoted unusually low is a pattern, and it is the mechanism through which an attractive quote becomes an expensive project.
This is why a contractor who priced conservatively upfront may cost less overall than one who quoted the optimistic case, and why asking each contractor what they have assumed is worth doing at the quoting stage.