most contractors price their decks per square foot. "the 3 biggest things that decide the cost of your deck are the size of your deck, the material you choose, and the options you add to it.". - john (owner) our average deck is around 400 square feet or 20' wide by 20' long.pricingidea startershow much does it cost to build a deck? | diywww.diynetwork.com/,/how-much-does-it-cost-to-build-a-deckadding a deck is a great way to expand your living area at a modest cost per square foot. according to the “cost vs. value report” from “remodeling” magazine, the national average cost to have a basic, all-pressure-treated wood deck professionally built is $31 per square foot. compare that to the national average cost for a two-story addition of about $210 per square foot, and outdoor living sounds really attractive.