Ask House Spouse · Exterior & Weatherproofing · Decks & Outdoor Structures
How much does a 20x20 deck cost in the Seattle area?
Short answer
In the Puget Sound region, a 20x20 deck (400 square feet) generally costs between $18,000 and $36,000 for pressure-treated wood or cedar, and $28,000 to $52,000 or more for premium composite with metal railings. Permitting, elevation, ground slope, and footing requirements in King and Snohomish County drive the final price.
The Major Cost Drivers for a 400 Square Foot Deck
A 20x20 footprint creates 400 square feet of usable outdoor living area, but the finished surface is only half the investment. Substructure framing, including concrete footings, 2x10 or 2x12 pressure-treated joists, beams, and structural hardware like hurricane ties, usually accounts for 30 to 40 percent of total project cost. Railing systems add another substantial line item. Code requires 36-inch railings on decks higher than 30 inches, and popular choices like aluminum balusters, cable railings, or glass panels can easily add $60 to $150 per linear foot over standard wood.
Cedar Versus Composite in the Pacific Northwest
In King and Snohomish County, moisture resistance defines your long-term return on investment. Western red cedar runs roughly $45 to $75 per square foot installed, offering natural beauty and rot resistance, but our wet climate demands rigorous cleaning and restaining every one to two years to prevent moss and cupping. Composite decking costs between $70 and $130 per square foot installed. While significantly more expensive on day one, composite will not splinter or rot, and it eliminates the regular staining cycles required by real wood.
Permits and Local Code Requirements
Most Puget Sound municipalities, including Seattle, Bellevue, and Everett, require a building permit for any deck over 30 inches above grade or any structure attached directly to the house. Permit fees typically run between $500 and $2,000 depending on valuation and city review times. Frost depth footings, earthquake-rated ledger connections, proper flashing to prevent moisture intrusion into your home's envelope, and graspable stair handrails are strictly enforced during framing and final inspections.
Rebuild Versus Resurface
Before committing to a $30,000 replacement, evaluate whether your framing can be saved. If the underlying posts, beams, and joists pass a structural inspection with no rot or sagging, resurfacing the deck with new cedar or composite boards costs roughly 40 to 60 percent less than a complete tear-out. For older decks with rotted ledgers or soft posts, a full rebuild is the only safe, code-compliant option.
Why it happens
Deck pricing in Western Washington comes down to framing labor, materials, and moisture protection. A 400 square foot deck requires substantial substructure, including concrete piers dug below the frost line, pressure-treated joists, ledger board attachment with flashing, and railing systems engineered to meet local building codes. In our marine climate, builders must also budget for corrosion-resistant hardware and protective joist tape to prevent rot from relentless winter rain. Material selection causes the widest pricing spread. Tight-knot Western red cedar remains a classic regional choice but requires regular sealing, while composite decking like Trex or TimberTech carries a much higher upfront material cost offset by lower maintenance. Complicated hillside terrain common in communities like Issaquah or shoreline setbacks in Edmonds further increase engineering and labor costs.
How to diagnose it
- 1Measure your proposed footprint and check whether grade changes require tall support posts or retaining walls.
- 2Inspect your house siding and rim joist where the ledger board attaches to verify whether dry rot exists.
- 3Check local jurisdiction building codes, as decks over 30 inches above grade require engineered permits and specific railing heights.
- 4Determine whether your existing framing is structurally sound enough for a simple board resurface rather than a full rebuild.
- 5Evaluate sun and tree cover in your yard, which directly influences whether cedar or composite makes better financial sense.
DIY vs. professional
DIY-friendly if…
Handy homeowners can comfortably tackle minor board replacements, power washing, and annual sealing on a low-to-the-ground deck.
Call a pro when…
Call a licensed contractor whenever work involves footings, structural framing, ledger board flashing tied into your home, or decks elevated over 30 inches.
Cost expectations
$18,000–$52,000typical WA labor + materials
Typical Puget Sound range for a full 400 sq ft build, from basic cedar on flat ground to high-end composite with custom railings.
Every House Spouse job is quoted in writing before we start. 12-month workmanship warranty included.
What we see on Home Health Assessments
Around 74% of PNW roofs we assess show active moss establishment — accelerates shingle wear by an estimated 3–5 years if untreated.
Caught early on assessment: $220–$650 · Left until failure: $3,500–$22,000
During a Home Health Assessment, we evaluate your existing deck framing, ledger board flashing, post footings, and structural connections to tell you if you need a full replacement or just targeted repairs and board replacement.
How the Home Health Assessment worksServices we'd bring to this job
Where we handle this
We answer this question on real Home Health Assessments across Snohomish and King County.