Deck sizes

The average deck size

Most residential decks land between about 200 and 400 square feet — a 12 by 16 at the small end, a 20 by 20 at the large. That band exists for a practical reason rather than a stylistic one: it is the smallest footprint that holds a dining set and a separate seating area while still leaving a walking route from the back door to the steps. Below about 150 square feet you are choosing between those two uses rather than having both.

Size drives almost everything else about a deck. It sets the material bill, the number of footings you dig, whether a permit is required, and whether the finished deck looks proportionate to the house. It is worth settling before you choose a material, because moving from pressure-treated pine to composite on a smaller deck often costs less than keeping pine and adding four feet.

Every deck size compared

Costs are installed, national averages, and cover framing, fasteners and labour. Building it yourself typically removes 40 to 55 percent. Click any size for its full material list — boards, joists, footings, concrete and screws.

How to choose the right size

1. Lay out the furniture first, not the deck. Measure the table and chairs you actually intend to use, add three feet behind each chair for pulling back and walking, and draw that rectangle. Everything else — grill, lounge chairs, planters — gets its own rectangle. The deck is the sum, not a round number you picked first.
2. Keep the route clear. The path from the back door to the steps should run along an edge, never through the middle of the seating. This single decision does more for how a deck feels than adding a few feet in either direction.
3. Watch the proportion to the house. A deck much bigger than about a fifth of the house's footprint starts to dominate the back elevation. It is a taste judgement rather than a rule, but it is the most common regret on oversized decks.
4. Check your setbacks before you settle on a number. Most municipalities require a deck to sit some distance — often 5 to 10 feet — from a property line, and that constraint frequently decides the depth for you.
5. Allow space for the stairs. A flight down from a three-foot deck needs roughly four feet of ground run plus a landing, and that space is outside the deck footprint. Work it out with the stair calculator before you site the deck.
6. Think about the permit threshold. If your jurisdiction exempts freestanding decks under 200 square feet and under 30 inches high, a 10 by 20 and a 12 by 18 are very different projects on paper even though they are a similar size.

Size, cost and the things that move the number

Cost scales with area, but not evenly. Fixed costs — the permit, the ledger, the stairs, getting a crew to site — spread over more square feet, so a larger deck is usually a little cheaper per square foot than a small one. Expect that saving to be in the region of 10 to 15 percent going from a small deck to a large one, not more.

Going the other way, three things add cost without adding usable area: height, shape and railing. A deck more than about three feet off the ground needs longer posts, deeper footings and guard rail everywhere. Angles, curves and level changes all cost framing time. And railing is priced per linear foot, so a long thin deck carries far more of it than a square one of the same area — a 10 by 20 has 40 feet of railing on three sides against 36 feet for a 12 by 16 that is only a little smaller.

Run your own footprint through the deck cost calculator for a figure that accounts for height, material and railing, or the cost per square foot calculator if you are comparing a contractor's quote.

Standard deck sizes and why they exist

The sizes in the table above are not arbitrary. Decking, joists and beams are sold in even lengths — 8, 10, 12, 16 and 20 feet — so a deck dimension that matches a stock length wastes almost nothing, while one that lands just over forces you to buy the next size up and cut the difference off every single board. A 16 foot deck built from 16 foot boards has no butt joints; the same deck at 17 feet needs a joint in every row, blocking under every joint, and produces a pile of offcuts.

That is why the jump from 16 to 20 feet costs proportionally less than the jump from 16 to 18. If your yard allows either, the stock length is usually the better buy.

Questions people ask about deck size