Getting three quotes is standard advice. What nobody explains is how to interpret the result when they come back far apart.
The most common reason: different jobs
Most large variation is not one contractor being expensive and another being cheap. It is that they have quoted different work.
One included the permit. One allowed for replacing rotten decking; another did not mention it. One priced restoring the siding disturbed during window replacement; another left it vague. One specified a mid-range product, another a premium one.
None of that is dishonest. It is what happens when three people are asked to price a job that was never defined precisely, and each fills the gaps according to their own habits.
Different assumptions about the unknown
In older Washington homes, a great deal of the eventual cost depends on what is found once work begins. Contractors handle that uncertainty differently.
Some price conservatively, building in allowance for likely findings. Others quote the optimistic case and address discoveries through change orders. The first looks more expensive on paper and may cost less in the end; the second looks attractive and can end up higher.
Neither approach is wrong, but they are not comparable unless you know which you are looking at. Ask each contractor what they have assumed.
How busy they are
A contractor with a full schedule prices differently from one with a gap next month. This is entirely rational and it produces real variation between equally competent firms.
It also means timing affects price. Washington’s seasonal compression in exterior trades — roofing, siding, decks — means quotes obtained in the dry-season rush differ from those obtained in the shoulder season.
Overheads and structure
A firm with an office, a supervisor and warranty administration costs more to run than a two-person operation working from a truck. The larger firm may deliver better project management, clearer communication and a warranty more likely to be honoured in five years. The smaller one may deliver equal work at lower cost.
Both are legitimate businesses. What you are choosing between is not quality but structure.
When cheap means something is wrong
A quote dramatically below the others usually means one of a few things: scope has been misunderstood or deliberately narrowed, cheaper materials are specified, an unregistered subcontractor is involved, or the intention is to recover the difference through change orders once you are committed.
Ask directly what accounts for the gap. A contractor with a real explanation will give one. A vague answer is the answer.
Making the comparison work
Write one brief and give it to everyone. Specify the breakdown you want. Ask each to state assumptions and give contingency rates. Then the numbers describe the same job and the spread means something.
With comparable quotes, a modest spread reflects overheads and scheduling. A large one still means something differs, and it is worth finding out what.