How much does a 16x20 deck cost in Boise?

What does a 16x20 deck cost in each material?

Expect four to five figures of difference between the cheapest and most expensive build of the same footprint. The table runs 320 square feet through the formula behind our online deck calculator for every material and height: published 2026 per-square-foot ranges, a height multiplier and a fixed spread, rounded to the nearest $500.

Height is a bigger lever at this size than on a small deck. A 320-square-foot composite deck moves from $10,500–$20,000 at ground level to $15,500–$29,000 off a second-floor door, because taller posts need bracing, the footings carry more and the stairs get longer. In Boise, any deck more than 6 feet high also needs stamped engineering under the city's #406 submittal checklist, which adds a design fee before anything is built.

What can you fit on a 16x20 deck?

Enough for two real zones and a walkway between them. That is the main reason people step up from a 12x16: dining and lounging stop competing for the same corner.

• Dining plus lounge: a six-seat dining zone takes about 12 x 9 feet with chairs pulled out, and a sofa-and-chairs group about 10 x 10. Laid side by side along a 20-foot house wall, they use roughly 16 x 20 with a narrow aisle, so this layout works best with the long side against the house.
• Dining plus grill plus traffic: put the grill at the far corner with the clearance its manual specifies, the table in the middle and a 36-inch path from the door to the stairs.
• Hot tub plus seating: a spa in the 7-to-8-foot class fits in one corner with room for a small table, but the spa needs its own engineered support (see below).
• Built-in benches or planters along the rail: they save floor space but add framing and are priced as custom work.

Orientation matters as much as area. With the 20-foot side along the house, joists run 16 feet and the deck feels wide and shallow, with more shade from the house in the afternoon on a west wall. Turn it so 20 feet runs out into the yard and you get a deeper deck that needs a second beam line, more footings and a longer walk to the far rail.

How is a 16x20 deck framed and how many footings does it take?

A ledger-attached 16x20 usually needs three or four footings under one beam when the 20-foot side is on the house, and six to eight when it is not. Here is how the 2018 IRC tables (Idaho's adopted residential code) play out for the first layout.

• Joists: a 16-foot clear run is near the limit of dimensional lumber. In Douglas fir-larch or hem-fir, 2x12s at 16 inches on center reach 15 ft 9 in, just short, while 2x12s at 12 inches reach 18 ft. The cheaper route is often a beam set about 3 feet in from the outer edge, so 2x10s at 16 inches span about 13 feet (allowed: 13 ft 7 in) and cantilever the last 3 feet, inside the code's one-quarter-span limit.
• Beam: 20 feet is too long for one span. In the code's southern pine column, a triple 2x12 beam under 14-foot joists spans 10 feet, so it needs a post at each end and one in the middle. Under 16-foot joists the same beam spans 9 ft 4 in, which pushes it to four posts.
• Footing size: with posts 10 feet apart and the beam 3 feet in, a middle post carries about 95 square feet of deck. At the IRC footing table's 1,500 psf soil column, up to 100 square feet calls for a 25-inch round or 22-inch square footing 8 inches thick, noticeably more concrete than a 12x16 needs.
• Frost depth: all of them go below Boise's 24-inch frost line.
• Free-standing or a 20-foot projection: add a second beam line and its posts. Expect six to eight footings and more digging.

Snow is not usually the governing load on a deck this size here. Boise's ground snow load is 20 psf, and the prescriptive deck tables are built on a 40 psf live load, which covers it. Our snow load guide explains where that changes, such as under a roof valley or a cover.

Does a 16x20 deck need a permit?

In most of the valley, yes, unless it is very low. Boise requires a permit for decks more than 12 inches above grade at any size, and so does Nampa. Meridian's exemption only covers detached decks of 200 square feet or less, so a 320-square-foot deck needs a Meridian permit even at ground level.

Covers and pergolas raise the bar again. Boise treats any attached patio cover as a permitted structure regardless of size, and a stand-alone pergola over 200 square feet also needs a permit and is usually treated as pole construction requiring an Idaho-licensed engineer. A pergola over the whole 16x20 is past that line. For the submittal itself, see the Meridian deck permit rules or the Boise guide linked from it.

What does a 16x20 price not include?

Our model leaves railing, stairs, old deck removal & haul-off, solid roof cover, pergola, low-voltage lighting out of the total and lists them as priced at your site visit. On a deck this size, railing is the one to watch.

• Railing: guards are required above 30 inches, 36 inches tall with openings that stop a 4-inch sphere. With the 20-foot side on the house, the open edge is about 52 linear feet (16 + 20 + 16) before the stair opening. That is about 30% more rail than a 12x16.
• Stairs: maximum 7¾-inch risers and minimum 10-inch treads, 36 inches wide, with a handrail at four or more risers. A deck 30 inches up takes four or five risers and a short run; a second-story deck needs a long flight or a landing.
• Hot tub support: engineered framing or a slab. The AWC DCA 6 prescriptive guide does not cover hot tubs.
• Demolition of an existing deck, a roof cover, pergola or low-voltage lighting.

Railing systems run from treated wood to cable and glass, and the spread per foot is wide. The railing cost guide breaks it down by system so you can add a realistic line to the platform number.

Is 16x20 the right size, or is it more deck than you need?

Size it to what you will put on it. A 16x20 earns its cost when you want dining and lounging at once, a grill, or a spa. If you only need one table, the 12x16 deck price breakdown shows the smaller build. For a sense of scale, the gap between the two sizes in raised composite is about $7,000 at the midpoints of each range, before the extra railing.

Material choice weighs more on a big deck because upkeep scales with area. Pressure-treated pine at 320 square feet means re-staining a large surface every two to three years in Boise's sun, which sees about 55 days a year at 90°F or hotter. Composite and PVC skip the stain but cost more upfront; in raised composite the midpoint gap over pressure-treated pine is about $6,000 at this size.

Making 16x20 quotes comparable

1. Which side goes on the house, and is it ledger-attached or free-standing.
2. Joist size, spacing and any cantilever; beam size and post count.
3. Footing count, diameter and depth.
4. Railing system and linear feet, stair count, and whether stair rail is included.
5. Decking brand, line and color; fastener type.
6. Permit, plan sheets and engineering if the deck is tall or carries a spa.

Line those up and the numbers start to mean something. When you want a measured figure, book an on-site estimate.