Bathrooms concentrate more regulated systems into a small space than any other room — plumbing, electrical, ventilation and waterproofing all in a few square metres. That makes the permit question worth getting right.
What usually does not need a permit
Replacing fixtures in their existing positions. New tiling. Painting. Replacing a vanity where the plumbing connections stay put. Swapping taps and shower fittings. These are finish-level changes and generally sit below the threshold.
What usually does
Moving the toilet, bath, shower or basin, because that relocates supply and waste lines. Adding a bathroom. Converting a half bath to a full one. Altering electrical, including adding circuits, moving outlets or installing new extraction. Removing or altering walls. Anything structural.
The moment plumbing moves, you are almost certainly in permit territory, and that is the single biggest determinant of both cost and complexity in a bathroom project.
Ventilation
Extraction is a code requirement and it deserves specific attention in Washington rather than being treated as a box to tick.
A bathroom in a marine climate is already operating close to the moisture level at which mold establishes. An extractor that is undersized, ducted into the attic rather than terminating outside, or never switched on will produce a mold problem here considerably faster than the same installation would in a drier state.
Three things to confirm: that the fan is sized for the room, that the duct terminates outside the building rather than in an attic or soffit, and that the run is as short and direct as practical. All three are cheap at installation and expensive to correct afterwards.
In Eastern Washington the requirement matters too, for a different reason. A fan discharging warm humid air into a cold attic produces condensation against the roof structure, which is arguably a worse outcome than the same mistake in Seattle.
Waterproofing
Not always a permit matter, but the element most likely to cause expensive failure later. Shower and wet area waterproofing is largely invisible once tiling is complete, and failures announce themselves through damage elsewhere.
Ask what waterproofing system is being used and how it is being detailed at the tricky points — corners, thresholds, and where the shower meets the floor. A contractor who answers specifically is a different proposition from one who answers vaguely.
Older homes
Opening a bathroom in an older Washington home frequently reveals conditions requiring attention: original wiring, galvanised supply lines, or framing damaged by a leak that has been running quietly behind the tiling.
Bringing the work area up to current standards is part of doing the job properly, and it is why a contingency in the budget is realism rather than pessimism in older stock.
Practical approach
Establish first whether you are moving plumbing, because that determines almost everything else. Have your contractor confirm required permits in writing and pull them. Confirm the ventilation specification explicitly. And budget a contingency if the house is older.