A 12x20 deck of pressure-treated pine costs about $12,300 built by a contractor at US-average prices ($7,200 to $17,300), which is $51 per square foot for 240 sq ft. Doing the work yourself leaves materials of about $4,250.
A 12x20 deck is a long strip along the back of a house: dining, grilling and seating in a row, with the 20 ft length matching the longest stock board.
12x20 deck cost by decking material
The same frame, footings, railing and steps, priced with three decking grades at US-average prices (HomeAdvisor, June 2026; Fixr, January 2023). The frame is always pressure-treated pine; only the boards change.
| Decking | Materials only | Built by a contractor | Typical per sq ft | Crew days |
|---|---|---|---|---|
| Pressure-treated pine | $2,100 to $6,400 | $7,200 to $17,300 (typical $12,300) | $51 | 7 |
| Cedar | $2,350 to $6,900 | $7,500 to $17,800 (typical $12,600) | $53 | 7 |
| Composite | $4,500 to $10,500 | $9,600 to $21,500 (typical $15,500) | $65 | 7 |
Every row includes a 3-footing foundation, a wood railing on 41 ft of open edge, 5 steps down from a 3 ft high deck and a building permit. Labor is HomeAdvisor's $15 to $35 per square foot, $25 typical. Your state moves the totals by its own wage and materials level, which the calculator above applies.
Material list for a 12x20 deck
The list below is the 12x20 deck as the framing engine lays it out: attached to the house with a ledger, 40 psf live load, Southern pine, 1,500 psf soil, 6×6 posts, 10 percent spare boards.
| Item | Quantity | Detail |
|---|---|---|
| 5/4 × 6 wood deck board (1 × 5½ in) | 29 boards | 20 ft long |
| Deck screws, No. 8 | 832 screws | 2 per board at every joist (IRC R507.7) |
| 2×8 joists | 16 pcs | 12 ft, cut to 12 ft |
| 2×8 rim joist | 1 pcs | 20 ft |
| 2×8 ledger board, treated | 1 pcs | 20 ft (at least 2×8, R507.9.1.1) |
| 2×10 for the beam | 3 pcs | 20 ft, 3 plies spliced only over posts |
| 6×6 posts, ground-contact treated | 1 pcs | 8 ft, cut to 1 ft 6 ½ in each (3 posts) |
| Post bases for 6×6 | 3 pcs | galvanized (ZMAX) or stainless |
| Post-to-beam connectors | 3 pcs | or notched posts with two ½ in bolts (R507.5.2) |
| Joist hangers for 2×8 | 16 pcs | one per joist at the ledger |
| Joist-to-beam ties | 16 pcs | one per joist at each beam |
| ½ in lag screws with washers | 16 pcs | staggered, 16.5 in on center (Table R507.9.1.3(1)) |
| Concrete mix, 80 lb bag | 9 bags | 4.5 cu ft for 3 footings |
| Tube forms, 10 in | 3 pcs | one per footing |
Cedar uses the same board count as pressure-treated pine (29 boards including spares). Composite changes the decking lines to 29 boards in 12, 16 or 20 ft lengths plus 432 hidden clips, and needs no deck stain, which the wood grades do. The deck board calculator shows the board layout row by row, and the deck calculator lets you change spacing, board width and pattern.
Framing for a 12x20 deck
With joists running the 12 ft width, the tables in IRC 2024 R507.6 select 2×8 joists at 16 in on center, 16 of them for the 20 ft length. The beam is a triple 2×10 on 3 posts spaced 10 ft apart (R507.5), each on a round footing about 20 in across (R507.3.1). The footing depth depends on your frost line, so check the deck footing calculator with your local depth, and the deck joist span table if you want a different lumber size.
Labor and time
Labor comes to $8,000 at the typical rate. At $70 an hour for each of two builders working eight hours, one crew day is $1,100, so the job is about 7 days of work for a two-person crew, not counting waiting for footings to cure or for the inspector. Add 5 stair steps with the stair calculator and the stringer layout is done before the boards arrive.
Does a 12x20 deck need a permit?
Under IRC 2024 R105.2 a deck is exempt from a permit only if it is no more than 200 sq ft, no more than 30 in above grade at any point, not attached to a dwelling and not serving the required exit door. A 12x20 deck is 240 sq ft, so the area test is not met. This page assumes the usual case, a deck bolted to the house 3 ft off the ground, which fails all three tests: 240 sq ft is over the 200 sq ft limit; 36 in above grade is over the 30 in limit; a deck attached to the house is never exempt. Budget the permit ($230 to $500 per HomeAdvisor) and the inspections. Many towns are stricter than the model code, so call the building department before you buy lumber.
12x20 deck compared with other sizes
Stepping down to 12x16 saves 48 sq ft and about $1,900 at the typical price. Stepping up to 16x16 adds 16 sq ft and about $740. Cost per square foot falls a little as the deck grows because the stairs, permit and ledger are fixed costs.
Next steps for a 12x20 deck
- Size every joist, beam and footing for a 20 by 12 ft deck in the deck calculator.
- Count boards by stock length with the decking calculator.
- Price a different shape, height or railing in the deck cost calculator.
- Add up the footings' concrete with the concrete bag calculator, and the finish with the deck stain calculator.
- Prices for every size are grouped in cost calculators.