Ask House Spouse · Exterior & Weatherproofing · Decks & Outdoor Structures
What does a 20x20 foot composite deck cost in the Seattle area?
Short answer
A 20x20 foot (400 sq ft) composite deck in the Seattle area typically costs between $20,000 and $40,000. The final price depends heavily on the condition of the existing substructure, the specific brand of composite material chosen, railing style, and site accessibility. Simple redecking is on the lower end, while a full rebuild is on the higher end.
Breaking Down the Costs: Materials vs. Labor
A new deck project cost is typically split close to 50/50 between materials and labor. Materials include composite boards, hidden fasteners, concrete for footings, and pressure-treated lumber for the entire substructure. Labor includes demolition of the old deck, hauling debris, framing the new structure, meticulously installing the decking and railings, and final cleanup. Because composite decking is more expensive and requires more precision than wood, both material and labor costs are higher.
Redecking vs. Full Rebuild: The Key Decision
The most significant factor in your budget is whether you can redeck or need to rebuild. Redecking involves removing the old deck boards and railings and installing new composite materials on the existing frame. This can only be done if the footings, posts, and joists are perfectly sound and properly spaced. A full rebuild is necessary if rot is present or the structure is undersized. It involves a complete teardown, pouring new footings, and building a brand-new frame, which can nearly double the project cost.
Why Substructure Quality is Critical in the PNW
In our rainy Washington climate, the substructure is everything. There is no point in putting a 30-year composite deck on a frame that will rot out in ten years. We always use pressure-treated lumber for all framing members and recommend applying joist protection tape to all horizontal surfaces. This waterproof barrier prevents moisture from soaking into the tops of the joists, which is the most common failure point. A well-built frame ensures your beautiful new deck will last for decades.
Don't Forget Railings, Stairs, and Permits
Beyond the deck surface itself, several other items impact the final price. Railings can add $30 to over $150 per linear foot depending on the material. A simple set of stairs might cost $1,000, while a more complex staircase with landings could be several thousand. Finally, most decks more than 30 inches off the ground require a building permit from your city (like Bellevue or Seattle), which adds fees for plan review and inspections.
Why it happens
The wide cost range for a composite deck is driven by three main factors: materials, labor, and structure. Composite decking itself is a premium material, often 2-3 times the cost of standard pressure-treated wood. Labor costs fluctuate based on whether the project is a simple redecking or a full teardown and rebuild, which requires new concrete footings and framing. In the Pacific Northwest, building a robust, rot-resistant substructure that will last as long as the composite decking is critical and adds to the overall investment.
How to diagnose it
- 1Check your existing deck's foundation and framing. Are the support posts, joists, and beams free of rot and structurally sound?
- 2Decide if you are simply redecking or doing a full rebuild. Redecking saves significant money but is only an option if the substructure is in excellent condition.
- 3Choose your decking material. Top-tier brands like Trex or TimberTech have different price points for their 'good, better, best' product lines.
- 4Select a railing style. Simple pressure-treated wood railings are the most affordable, while aluminum, cable rail, and glass panel systems increase the cost considerably.
- 5Assess site access. A sloped, difficult-to-access backyard in a hilly neighborhood will increase labor costs for demolition and material transport.
- 6Determine if a permit is needed. In Washington, decks over 30 inches above grade typically require a building permit, which involves design, review, and inspection fees.
DIY vs. professional
DIY-friendly if…
DIY is suitable for replacing a few individual composite boards or performing annual cleaning and maintenance on an existing deck.
Call a pro when…
A professional general contractor is required for a full redecking project or a new build to ensure structural integrity, proper water flashing against the house, and code compliance.
Cost expectations
$20,000–$40,000typical WA labor + materials
Cost covers demolition, labor, and materials for a 400 sq ft deck. A simple redeck is at the low end; a full rebuild with premium materials is at the high end.
Every House Spouse job is quoted in writing before we start. 12-month workmanship warranty included.
What we see on Home Health Assessments
Roughly 71% of homes older than 8 years show one or more failed caulk joints letting water into wall assemblies.
Caught early on assessment: $220–$750 · Left until failure: $1,200–$8,500
A Home Health Assessment is perfect for evaluating the condition of your existing deck's framing and footings. This inspection helps you determine if you can save money by redecking or if you need to budget for a full, more costly rebuild.
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.