A 400-square-foot deck is the sweet spot between functional outdoor space and a project that doesn’t require a small construction crew. Good 20×20 deck plans account for joist span, post spacing, frost-line footings, and code-compliant railing — the kind of details that turn a weekend dream into a 10-day reality with permit headaches if skipped. This guide covers the structural choices, layout options, and realistic budgets for a build that lasts 25 years or more.
Part of our Decks & Outdoor guide — explore the full guide →
Footings and Frame Layout
For a freestanding 20×20 deck at standard residential height (under 30 inches off grade), most plans use a 6-post or 9-post layout. The 9-post option uses two rows of three posts inside the perimeter on 10-foot centers, with a perimeter beam supported on cantilevered ends. The 6-post layout uses two rows of three posts running parallel to the house, leaving more open visual space underneath.
Footings must extend below the local frost line — 12 inches in zones like North Carolina, 42 to 48 inches in Minnesota, Maine, or upstate New York. Use 12-inch diameter Sonotubes minimum, with a flared base for any deck supporting more than 6 feet of joist span. Each footing requires roughly 2 to 3 cubic feet of concrete; a 20×20 with nine footings burns about 25 80-pound bags total.
Joist Sizing and Spacing
Joist size depends on span. For a 10-foot joist span (typical with the 9-post layout), 2×10 pressure-treated joists at 16 inches on center work fine. For 12-foot spans, jump to 2x12s. Cut spacing to 12 inches on center if you plan to install diagonal decking, which adds 30 percent stress to each joist.
For composite decking like Trex or TimberTech, manufacturers usually require 16-inch spacing or tighter for straight runs and 12-inch for diagonal patterns. Check the specific product specs before framing — getting it wrong voids the warranty.
Decking Material Choices
- Pressure-treated pine: $1.50 to $3.00 per sq ft installed; needs annual cleaning and resealing every 2-3 years
- Cedar: $4 to $7 per sq ft; naturally rot-resistant but soft and dents easily
- Composite (Trex, TimberTech, Fiberon): $7 to $14 per sq ft; 25-year warranty, no staining required
- PVC (Azek): $10 to $18 per sq ft; lightest and most stain-resistant
- Tropical hardwoods (ipe, cumaru): $12 to $25 per sq ft; 50+ year life but requires pre-drilling and special fasteners
For a 400-square-foot surface plus 12 to 15 percent waste, plan to buy 450 to 460 square feet of decking material. Composite is the most popular pick for new builds because the maintenance is essentially zero.
Three Layouts to Consider
The first option is a straight square deck attached on one 20-foot side to the house, with stairs centered on the opposite side. Simple, code-easy, and the cheapest to build. Joists run perpendicular to the house, decking parallel.
The second is a freestanding deck a few feet off the house, connected by a short bridge or pergola walk. This avoids ledger board complications, simplifies waterproofing, and lets you orient toward a view rather than the back wall.
The third is the multi-level approach: a 20×14 main deck stepped down to a 20×6 lower platform. Adds visual interest, defines zones (dining vs. lounging), but requires more framing and an additional set of stairs. Budget another 15 percent for the multi-level option.
Stairs and Railings
Stairs require a 7.5-inch maximum riser and 10-inch minimum tread depth per IRC 2021. For a deck 30 inches above grade, you’ll need 4 risers and 3 treads, with a 30-inch run from deck edge to ground.
Code requires 36-inch minimum railing height when the deck sits 30 inches or more above grade. Spindles must be spaced so a 4-inch sphere cannot pass through. For a 20×20 deck with stairs, that totals about 80 linear feet of railing — a budget item that surprises a lot of first-time builders. Aluminum balusters with cedar or composite top rail run $30 to $60 per linear foot installed.
Permits and Code Compliance
Almost every municipality requires a permit for a deck attached to the house or larger than 200 square feet. Expect $80 to $400 in permit fees, an inspection of the footings before pour, a frame inspection before decking, and a final inspection after railings. Skipping permits voids most homeowner’s insurance claims if anyone gets hurt and creates problems at resale.
If attaching to the house, the ledger board is the most code-scrutinized component. Use through-bolted ½-inch lag screws with washers on 16-inch centers, flash with self-adhered membrane and stainless drip cap, and verify the rim joist behind the siding is sound. Improper ledger attachment causes more deck collapses than any other failure point.
Budget Ranges
For DIY with pressure-treated lumber and standard wood railing: $4,500 to $7,500 in materials. For DIY with mid-grade composite and aluminum railing: $9,000 to $13,000. Hire a contractor and the labor doubles those numbers, putting a finished 20×20 composite deck in the $18,000 to $30,000 range depending on region and finish quality.
Allow 7 to 14 days for a confident DIYer working weekends, or 4 to 7 working days for a 2-person crew.
Common Mistakes
Overlooking lateral load connectors at the ledger is the big one — code now requires hold-down brackets every 8 feet. Skipping joist hangers at the rim, using interior-grade screws, and notching posts past the allowable depth are the next three. Always use ACQ-rated fasteners and hangers with pressure-treated lumber, since the copper in modern PT will corrode standard galvanized steel within 3 to 5 years.
Plan the layout on paper, verify the footing locations against utility lines via 811 before you dig, and walk the framing one more time before screwing down decking. A solid 20×20 deck is one of the best home improvement returns you can make — about 65 to 75 percent recovered at sale, plus years of usable square footage.
