12×20 Deck Cost

A 12×20 is 240 sq ft. The decking surface runs about $6,000–$9,600 installed in pressure-treated and $12,000–$19,200 in mid-range composite at 2026-Q1 national rates. The complete project at the calculator's default spec — railing, four steps, permit and a 10% contingency — is near $27,100, which works out at $113 per square foot. That is $26 per foot below a 10×10 on a deck nearly two and a half times the size, and almost none of the improvement comes from cheaper materials. It comes from the permit, the stair flight and the railing being spread across more floor: fixed costs are about 23% of the budget here, against 36% on a compact deck.

Yes, essentially everywhere. IRC 2021 R105.2 sets its permit-exemption ceiling at 200 sq ft, and a 12×20 is 240 — so the exemption does not apply regardless of how low the deck sits or whether it attaches to the house. That is not a reason to build smaller. The permit is a minor line on a $27,000 project, and what it buys is an inspector looking at the ledger connection and the footing depth, which are the two details that actually fail on decks. What it does change is the schedule: plan review takes days to weeks, footings must be inspected before concrete is poured, and framing has to clear inspection before decking covers the joists. Build the calendar around those holds rather than around the delivery date.

SPF 2×10 joists at 16 inches on centre, one beam line, and just 4 footings. Joists span the short dimension, so they run 12 feet here — inside the 13 feet 0 inches that 2×10 SPF carries at 16 inches on centre in the IRC 2021 R507.6 table, with a foot to spare. That means a 240 sq ft deck sits on the same four-footing foundation as a 12×12 less than two-thirds its size, with 16 joists and 40 linear feet of beam doing the rest. It is worth comparing with a 16×16, which is only 16 sq ft larger but needs two beam lines and 6 footings because a 16-foot joist run cannot be spanned economically.

Twenty-six rows of 20-foot boards — 36 boards and roughly 576 linear feet including waste. The 12-foot width divided by the 5.625-inch effective row width gives 26 rows, each covering the full 20-foot length in a single piece, because 20 feet is the longest decking length sold in both wood and composite. No butt seams, no offcut, and no sistered joist to carry a seam. Do plan the delivery, though: 20-foot boards need a long truck or a trailer, they flex badly when carried by one person, and composite in particular scuffs if dragged. If the yard allows it, have them dropped on the build side of the house rather than in the driveway.

A two- or three-person crew typically takes three to five working days on a 240 sq ft rectangle: a day for layout and footings, a day or two for framing, a day for decking, and a day for railing and stairs. That is build time, not project time. Add the permit review before any of it starts, plus two inspection holds — footings before the concrete goes in, framing before the decking covers it — and the calendar usually runs two to four weeks end to end. A DIY build on weekends is realistically six to ten days of work, and the 20-foot boards are the part where a second pair of hands stops being optional.