Washington · Pergola guide
How to build a pergola in Washington
4-post pergola with rafters and optional slats. A 4-post pergola frames a patio with shade and structure — no roof, minimal permit exposure, huge visual payoff.
Typical cost
$1,350–$2,144
Time
1–2 weekends
Skill
Beginner-friendly
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Permits & code in Washington
Open pergolas without a roof are treated as accessory structures — usually no permit under 120 sq ft, but setback rules still apply. Frost depth here is 18". Seismic bracing requirements apply to sheds and garages in the western half of the state.
State-level rules only — cities and HOAs frequently add stricter requirements. Always confirm with your local building department before you dig.
What most people get wrong
- Bolting rafters through the post — a lag from the top splits over time. Notch it.
- Setting posts without a batter board — the pergola comes out a parallelogram.
- Forgetting expansion gap on slats. They cup within a year.
The 5-step pergola plan
- Step 1Confirm site and permit pathLocate property lines, call 811 for utility marking, and check whether your city/HOA needs approval before you break ground.
- Step 2Order materialsUse the shopping list from our planner. Confirm delivery vs pickup — bulk lumber and concrete are usually cheaper delivered.
- Step 3Lay out and prep the siteBatter boards, string lines, level the base. Getting this right saves days later.
- Step 4Frame the structureFollow the cut list. Check for square with the 3-4-5 method every few pieces. Fasten with the right structural screws.
- Step 5Finish and inspectSand, stain/seal, and (if required) schedule your final inspection before backfilling or closing anything up.
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