Plumbing permitting sits between electrical, where almost everything is permitted, and cosmetic work, where almost nothing is. The line falls in a reasonably intuitive place.
The dividing principle
Replacing something where it already is generally does not require a permit. Moving it, adding to it, or replacing the pipework itself generally does.
Swapping a toilet for a new one in the same position: usually not. Moving that toilet to the other side of the bathroom: yes. Replacing a tap: no. Repiping the house: yes.
What typically requires a permit
Relocating supply or waste lines. Adding fixtures, particularly where it increases the fixture count. Whole-house repiping. Sewer line replacement. Water heater replacement in some jurisdictions, particularly where the installation changes. Adding a bathroom. Work involving the water service entering the property.
As with roofing, the specific thresholds are local. A jurisdiction may permit water heater replacement while a neighbouring one does not.
Why the inspection matters here
Plumbing failures share the quality that makes electrical inspections valuable: they are hidden and delayed. A poorly made joint inside a wall cavity can weep for years, and the first evidence is often damage to something else entirely — a stained ceiling, a soft floor, a musty smell.
In Washington’s marine climate that hidden moisture is more consequential than it would be in a drier state, because materials dry slowly and mold establishes readily. An inspection before the wall closes is worth having.
Sewer lines
Sewer replacement generally requires permits and involves work in the right of way, which brings additional requirements. It is also work where getting a second opinion is particularly worthwhile.
Ask whether a camera inspection was performed before any recommendation to replace a sewer line. Recommending replacement without one is guesswork, and the difference between clearing root intrusion and replacing a full run is substantial.
Septic properties
If your property is on septic rather than sewer, work affecting fixture count or bedroom count may involve the health department alongside the building department, because system capacity is a design consideration.
This is common in rural Pierce, Kitsap, Skagit, Whatcom and Yakima areas. Raise it at design stage rather than after plans are complete.
Certification too
As with electricians, Washington plumbers hold individual certification alongside their employer’s contractor registration. That gives you two things to verify rather than one, and it is worth using on any substantial plumbing work.
The practical question
Ask your plumber directly whether the work requires a permit at your address, who is pulling it, and whether it is in the quoted price. Get it in writing. If the answer is that no permit is needed, having that in writing too is a reasonable thing to request.