A 20 by 20 deck covers 400 square feet, and the price spread on that footprint is wider than most homeowners expect. Two quotes from one deck builder to the next can differ by more than $10,000 on the same square footage.
Material grade, framing height, stair count, and soil conditions each move the number. The breakdown below shows where the dollars land on a 400 square foot build in western Washington.
What 400 Square Feet Typically Costs
Installed pricing in the Puget Sound region sorts by material system more than by size. SmartDecks prices most 400 square foot projects inside these bands:
- Pressure treated frame and pressure treated decking: $30 to $50 per square foot, or $12,000 to $20,000
- Pressure treated frame with capped composite decking: $55 to $90 per square foot, or $22,000 to $36,000
- Steel framing upgrade: adds $8 to $12 per square foot
- Cedar decking over treated framing: $45 to $70 per square foot
Most projects land near the middle of a band rather than at either edge. The low end assumes a ground level platform with no stairs and no railing. Lighting, built in benches, and roof structures are priced separately from the deck itself.
Where the Money Actually Goes
The per square foot figure hides how uneven the spending is. Component costs on a 400 square foot build:
- Concrete footings: $150 to $400 each, with nine to twelve typical
- Framing lumber: $2,800 to $4,500
- Decking material: $1,600 at the low end, $7,000 or more for premium composite
- Railing: $70 to $140 per linear foot installed
- Stairs: $180 to $300 per step, including stringers and treads
- Hidden fasteners and joist tape: $500 to $1,200
- Labor: 45 to 60 percent of the contract total
- Permit and plan review: $250 to $700 depending on jurisdiction
- Disposal and site cleanup: $300 to $800
Framing Sets the Real Price
Decking gets the attention. The structure underneath sets the cost. Framing decisions that never show on the finished surface include:
- Joist spacing: 16 inches on center for straight decking, 12 inches for diagonal patterns
- Ledger attachment: through bolts or structural screws, never nails
- Ledger flashing: the single detail that prevents most premature rot
- Joist tape: a few hundred dollars that adds years of framing life
- Post spacing: wider spacing means larger beams and deeper footings
- Beam type: dropped beams cost less than flush beams set inside the frame with hangers
Homeowners comparing bids from more than one deck builder for deck installation often find the gap between quotes sits almost entirely in this category.
Washington Load Rules Change the Math
The state residential code sets deck live load at 60 pounds per square foot, half again higher than the 40 psf used across most of the country. A 400 square foot deck is sized for 24,000 pounds of live load plus its own weight.
What that means for pricing:
- Beams and posts size up compared with the same deck built in another state
- Post spacing on a 20 foot beam line often lands at six to seven feet
- Footing diameter grows with the load each post carries
- Guardrails above 30 inches must resist 200 pounds of lateral force
- Baluster gaps must block a 4 inch sphere, and guards run 36 inches minimum
Permits and Inspections in Kent
Kent exempts a deck from permitting only when it is under 200 square feet, no more than 30 inches above grade, freestanding, and not serving a required exit door. A 400 square foot build fails the first test, so a permit applies regardless of height.
Permit stage basics:
- Plan review and permit fees commonly total a few hundred dollars for a residential deck
- A site plan showing dimensions and setbacks is required with the application
- Inspections happen at least twice: footings, then framing before decking goes down
- Reinspection slots book out during summer, so a failed inspection can cost a week
Checking the Contractor Before the Contract
Every registered deck builder in Washington carries a surety bond and liability insurance, and the record is public. Bond amounts rose on July 1, 2024:
- General contractors: $30,000
- Specialty contractors: $15,000
Bond history, registration status, safety citations, and lawsuits against the bond all appear in the Department of Labor and Industries verify tool. Bids that leave out permit costs often leave out registration too, and unpermitted decks surface later during home sales.
Site Conditions That Move the Estimate
SmartDecks crews price site conditions during the walkthrough, because a footing surprise in week two is the most common source of change orders on decks this size. Elevation is the largest variable after material. A ground level platform needs no railing and no stairs. The same deck at eight feet adds guardrails on three sides, a full stair run, taller posts, and lateral bracing, which can add $6,000 to $9,000.
Other conditions that change the number:
- Slope: stepped footings and varied post lengths
- Soil bearing: soft or fill soil calls for wider footings or pier extensions
- Footing depth: at least 12 inches below undisturbed grade in most King County jurisdictions
- Demolition: removing an existing deck runs $900 to $2,200 with disposal
- Access: tight side yards slow material delivery and add labor hours
- Utilities: gas lines or irrigation under the footprint require rerouting
Reading Two Quotes Side by Side
A useful bid from any deck builder names joist spacing, beam size, footing count and diameter, fastener type, railing system, and warranty terms. Quotes priced only by square foot leave room for substitutions after signing. A $4,000 gap between two proposals usually traces to one material line or one framing decision rather than to markup.
Timeline follows scope:
- Ground level 400 square foot build: four to seven working days once permits clear
- Elevated deck with stairs and railing: eight to twelve working days
- Permit review in most King County cities: two to four weeks before the first shovel
Our team handles custom deck design and construction across King and Pierce counties, and span calculations and footing layout happen at the estimate stage rather than after framing starts.
What a Site Measurement Settles
Published averages give a starting range. A 20 by 20 price firms up only after someone measures ledger height, checks soil, and counts the footings the design needs. That visit runs about 45 minutes on a standard residential lot and resolves most of what separates a $14,000 project from a $28,000 one.
Homeowners weighing a 400 square foot build can reach SmartDecks at 253-234-7373 for a measured estimate covering material comparisons, elevation, jurisdiction specific permit requirements, and a written breakdown of the framing package.
