Deck Cost per Square Foot in Seattle
The full per-sqft decomposition for Greater Seattle — materials, labor hours, railing system, permit fees, and WA 10.35% sales tax — built on a 500 sqft worked example with 2026 reality applied.
2026 Deck Cost Estimator
Directional range based on material tier, slope condition, jurisdiction, and current site state. Built from 2026 PNW dealer-installed cost data.
$18,400 – $46,000
Range covers materials and labor before permit fees, Washington 10.35% retail sales tax, and any project-specific engineering. We refine to a single open-book number after site visit and structural review.
Seattle SDCI valuation-based; SFI permits ~40% of plan review fee. Typical deck permit fee band runs $400-$1,800; ECA-overlay decks add geotech review costs separately. 2025-2026 saw inflation-adjusted hourly rates rise materially.
Range includes January 2026 Trex/TimberTech repricing — composite Signature and Transcend lines were lifted +7-15% YoY per manufacturer notices. Cost guides published before Q4 2025 do not reflect this adjustment.
Estimates are directional only. Final pricing requires site visit, structural review, and permit feasibility check. Slope and ECA conditions in particular shift the band substantially after stamped engineering enters the scope.
Installed cost per sqft by material and slope
Per-sqft is a compressed metric. It mixes substructure, boards, railing, stairs, labor, and the regulatory layer into one number. Use it for first-pass comparison, then move to the line-item breakdown below. PNW installed bands draw on 2026 national installed-cost medians (HomeGuide / Angi) and PNW dealer-installed benchmarks and apply the January 2026 Trex/TimberTech repricing.
| Material | Flat lot $/sqft | Slope / ECA $/sqft | Note |
|---|---|---|---|
| Pressure-treated pine | $25 – $50 | $40 – $80 | Substructure rebuild and helical piles drive the slope band higher. |
| Western Red Cedar | $30 – $90 | $45 – $130 | PNW-installed premium; reseal every 1-3 years not included. |
| Standard composite (Trex Enhance / Select) | $40 – $100 | $65 – $150 | Most-quoted tier in Greater Seattle outside ECA. |
| Premium capped composite (Trex Transcend Lineage) | $100 – $130 | $120 – $190 | Includes January 2026 +7-15% manufacturer repricing. |
| Capped polymer PVC (TimberTech AZEK Vintage) | $90 – $150 | $120 – $220 | Waterfront, shoreline, deep-shade contexts. |
| Thermally modified ash / garapa | $110 – $180 | $140 – $260 | Ipe alternative; CITES-restricted Ipe excluded from default scope. |
Sources: HomeGuide / Angi 2026 installed-cost data; PNW dealer-installed benchmarks; January 2026 Trex repricing.
Why 2026 per-sqft is higher than 2024 per-sqft
The per-sqft number has moved upward in the last 18 months for reasons unrelated to contractor margins. Four shocks layered into the market between Q1 2025 and Q2 2026, each documented and each visible in line-item breakdowns from open-book contractors.
Permit fees first. King County (unincorporated) raised deck permit fees +49% on January 1, 2025 (Ord. 19857), another +~14% on January 1, 2026 (Ord. 20021), plus a new $126 application screening fee. On a 500 sqft project that shifts the permit line from roughly $1.80/sqft in 2024 to $3.80-$4.80/sqft today.
Materials second. Trex repriced railing, fasteners, and Signature/Transcend lines +7-15% effective January 1, 2026. TimberTech AZEK followed in parallel. On a 500 sqft Trex Select build the boards alone moved from roughly $6.00/sqft materials to $7.00-$8.00/sqft.
Lumber third. Q2 2026 framing lumber rose +5.11%, national average ~$916/MBF in April. PNW labor and supply constraints amplify the move — the substructure line item carries the increase regardless of finish material.
Code fourth. Washington adopted the 2021 IRC effective March 15, 2024. WA-specific 42-inch guard requirement (vs IRC 36-inch baseline) and Section R507.10 continuous-load-path requirements shift railing system selection and stamped-engineering scope on any deck above 30 inches.
500 sqft Trex Enhance Naturals build, Seattle flat lot
Representative 500 sqft project: tear-out of an aging 480 sqft cedar deck, replacement with Trex Enhance Naturals on concrete pier substructure, composite Enhance railing at the WA 42-inch spec, one stair flight, Seattle SDCI permit pathway. Per-sqft and total cost columns shown side by side.
- Line itemDetailPer sqft500 sqft
- Demolition & disposalTear-out of existing 480 sqft cedar deck, transfer fees$5 – $9$2,500 – $4,500
- Footings & substructureConcrete piers, 60-psf pressure-treated framing, R507.10 load path$18 – $24$9,000 – $12,000
- Boards (Trex Enhance Naturals)500 sqft at $7-$9/sqft materials (Jan 2026 price sheet) + 10% waste$7 – $9$3,500 – $4,500
- Composite railing (42")60 linear ft Trex Enhance railing at WA 42-inch spec$7 – $11$3,600 – $5,400
- StairsEngineered stringers, composite treads, code-compliant landing$4 – $6$2,000 – $3,000
- LaborPNW licensed carpenter crew, ~170 hrs at $50/hr blended$17$8,500
- Seattle SDCI permitValuation-based SFI permit + inspections per 2026 fee code$2 – $3$1,000 – $1,600
- WA 10.35% sales taxApplied to materials portion (Seattle/King combined retail rate)$3 – $4$1,500 – $2,000
- Total — directional range$63 – $86$31,600 – $43,000
Materials weighted to Trex’s January 2026 price sheet. Labor at PNW carpenter $50/hr blended. Permit cost per the SDCI 2026 fee code. Sales tax at WA Department of Revenue Seattle/King combined retail rate.
2026 Permit Fee Matrix
Four core Greater Seattle jurisdictions, their deck permit thresholds, and the 2026 fee schedules behind every quote you receive.
| Jurisdiction | Permit Threshold | 2026 Fee Structure | Recent Change |
|---|---|---|---|
| Seattle SDCI | >18" above grade; any roof deck; any deck in an ECA overlay | Valuation-based sliding scale. SFI (Subject-to-Field-Inspection) permits run ~40% of the plan review fee. 2026 Fee Code and Fee Estimator published. | +12-19% in 2025, +18% inflation-adjusted hourly rates in 2026 |
| King County (unincorporated) | >30" above ground (uncovered decks at or below 30" outside critical areas exempt) | $772 application review + $2.46/sqft inspection (deck of 500 sqft or less) + new $126 application screening fee | +49% on Jan 1 2025 (Ord. 19857), +~14% on Jan 1 2026 (Ord. 20021) |
| Bellevue | >30" above ground (plan review included in permit) | Valuation-based. Reported by local contractors as relatively faster and less painful than King County unincorporated. | No major fee-schedule shock reported through Q2 2026 |
| Pierce County (unincorporated) | >30" above adjacent grade; guardrail required on all decks >30" | $69 for the first $2,000 of valuation + $12.21 per additional $1,000 up to $25,000. Fast Track available for pre-designed single-level decks up to 14 ft high. | Updated effective Feb 1, 2026 (Pierce County Code 17C.10.070) |
Source: Seattle SDCI 2026 Fee Code; King County Ord. 19857 / 20021; Bellevue Development Services; Pierce County Code 17C.10.070. Threshold and fee data current as of Q2 2026. Final permit cost on any project depends on valuation, site conditions, and whether structural engineering or geotechnical review is triggered.
Frequently asked
- What is the average deck cost per square foot in Seattle?
- On a flat lot, $40-$100/sqft installed for standard composite is the central band most homeowners encounter. On slope, ECA, or premium-material projects the band runs $100-$190/sqft installed. The arithmetic mean of the Greater Seattle composite cost band sits around $75-$85/sqft installed for non-slope work.
- Why is the per-square-foot range so wide?
- Because per-sqft compresses six distinct line items into one number: demolition, footings, decking boards, railing, stairs, labor. A 200 sqft deck on a steep slope with cable railing has a different per-sqft cost than a 600 sqft deck on a flat lot with the same boards. Per-sqft is useful as a rough comparison; the worked example below decomposes it line by line.
- Do I need a permit at $40/sqft or only at $120/sqft?
- Permit threshold is structural, not financial. Seattle requires a permit on any deck more than 18 inches above grade, any roof deck, and any deck in an ECA — regardless of material cost. King County, Bellevue, and Pierce County use 30 inches. A $40/sqft pressure-treated build 24 inches off the ground in Seattle requires the same permit as a $120/sqft Transcend Lineage build 24 inches off the ground.
- How much does the WA 42-inch guard requirement add per square foot?
- On a 500 sqft deck with a 60-linear-foot perimeter, the upgrade from IRC 36-inch to WA 42-inch railing typically adds $300-$900 of materials at the system level (cable, glass, and aluminum systems price per linear foot at the taller spec). That works out to roughly $0.60-$1.80 per square foot of deck — meaningful on premium systems, negligible on basic wood guards.
- Why does the 2026 number not match the cost guide I read last year?
- Four discrete shocks landed between Q1 2025 and Q2 2026: King County permit fees +49% then +14%, Seattle SDCI hourly rates +12-19% then +18%, Trex/TimberTech composite line repricing +7-15% effective Jan 1 2026, and Q2 2026 framing lumber +5.11%. Cost guides republished before Q4 2025 do not reflect these. The per-sqft math has to be recomputed for any quote pulled after January 2026.
- Is there a way to reduce per-sqft cost without compromising the structure?
- Two legitimate levers. First, resurface rather than replace if framing passes structural review (40-60% savings). Second, design to the existing footing layout where possible — the substructure is the biggest single line item and the most expensive to relocate. A $40-$60/sqft band is reachable on resurface scopes where the framing inspection passes.
Per-sqft is a starting point. The quote is the document.
We don’t quote at a flat “$X/sqft” rate — the math collapses too many variables. Each project receives a line-item open-book quote with the SDCI or county permit tracking number on day one, current manufacturer price sheets attached, and PNW carpenter labor hours posted.