The venomous question, answered plainly
This is the question everyone actually has, so here it is without hedging. Washington state pest data names one venomous snake, the western rattlesnake, and places it on the eastern side of the Cascades. Seattle sits west of that divide. That is a genuinely reassuring fact, and it is also a limited one. It tells you which venomous species exists in the state and roughly where it is found. It does not tell you what any individual animal in your yard is, and it is not a license to pick something up because the odds look good. Odds are a poor basis for putting a hand into a hedge. The practical result is a calmer version of the same procedure used anywhere. Back off a few feet. Photograph the body along its length rather than the head, since head shape is deliberately misleading in frightened harmless snakes. Send the picture to the Washington Department of Fish and Wildlife or a licensed wildlife operator. Keep pets indoors until you have an answer. In the great majority of Seattle cases the reply will be the name of a harmless colubrid, and the correct next action will be to do nothing at all except get the boards off your lawn.
What this page cannot tell you, and who can
Being straight about the gaps matters more on a snake page than on most. The data behind this page does not include a full species list for Washington, a set of state regulations, or bite statistics specific to the state. Rather than fill that space with plausible sounding detail, it is better to say so and point you somewhere real. The Washington Department of Fish and Wildlife is the authority for which species occur where, which are protected, and what a homeowner is permitted to do. If you want a species named, that is the door to knock on, along with any licensed wildlife control operator working in the area. This matters practically, not just as a disclaimer. Snake advice on the internet is full of confident species lists that were written for another state, and acting on the wrong list is how people end up killing a harmless native animal or, less often but more seriously, misjudging one that deserved distance. What this page can tell you is the part that does not change with the state line: how to photograph safely, what the quarter inch exclusion tolerance means for a building, why rodents drive the whole thing, and which products do nothing at all.
Why a wet climate builds the food chain in your beds
Thirty eight inches of rain a year and 73 percent average humidity produce a garden that never really dries out, and that is the engine behind most snake sightings around Seattle. Start at the bottom. Damp soil and heavy mulch support slugs, worms and insects in quantity. Those support frogs and small mammals. Vole runs appear under long grass and in ivy, and mice work the compost and the shed. A snake arriving in that setting is not lost; it has found a working buffet and is staying for it. Cover is the other half. Ivy at a wall base, blackberry along a fence, moss thick enough to hide movement, stacked pots, and above all flat material lying on wet ground all provide the combination of shelter and warmth that a cold-blooded animal needs in a mild climate. A sheet of plywood on grass may be the single most attractive object in a Seattle yard. The useful conclusion is that habitat work outperforms everything else here. Lift the flat stuff, thin the ivy, keep a bare strip along the foundation, and deal with the rodents. You will see fewer snakes, and you will see them further from the door.
Crawlspaces, vents and the quarter inch rule
The exclusion threshold UC IPM uses is any opening greater than a quarter inch, roughly a pencil width, and it is the same tolerance used to keep rodents out. That single number converts a vague worry into a walkable checklist. In Pacific Northwest housing the crawlspace does most of the damage. Foundation vents are commonly screened with insect mesh that rusts through in damp air or gets pushed out at one corner, and a torn vent is no barrier at all. Skirting and lattice look sound from the street and are often loose along the ground line. The access door warps in a wet climate, stops latching, and sits ajar for a whole season while nobody notices. Work outward from there. Garage door bottom seals flatten with age. Dryer vents, hose bib collars and utility penetrations leave rings of unsealed gap. Basement doors sit above worn thresholds. Fix these with hardware cloth stretched across whole openings rather than patched over holes, sealant at penetrations, and hardware that latches. Do the inspection before the sealing, especially in a mild climate where something may already be under the house, and do it before autumn rather than after.
Start with the rodents, not the snake
If you take one operational idea from this page, make it this: a snake sighting is information about your rodent population. Snakes do not eat garbage, plants or pet kibble. They eat voles, mice, slugs, frogs and lizards. An animal that crosses a property and stays has found a food supply worth staying for, so the sighting is a symptom and the rodents are the condition. This is why removal on its own so often disappoints. Take the snake away and the buffet is untouched, so another animal finds the same yard within weeks. In a Seattle garden the rodent supply is usually easy to trace. Long grass and ivy give voles cover and runs. Open compost feeds mice. Birdseed and pet food stored in bags rather than sealed bins feed everything. Sheds with clutter on the floor provide nesting. UC IPM's habitat advice points at the same target from a different direction: remove brush, rock piles and rodent burrows. Do that work first and the snake question usually answers itself, because a yard that stops feeding rodents stops being worth a visit.