Building a deck takes work and energy, but you can often design your deck to be larger without it being much more work. Doubling the size of your 12x12 won't take twice as much energy, since you're only adding one or two more footings, two more railing panels, and 8 more joists. You're still putting up one ledger, one beam, and borders on 3 sides.
Deciding on the size of your deck is more than just how much material you can afford.
Home Width and Layout
Although you can build a deck that's wider than your house, it is usually easier to keep it the width of the back side of your house. Your patio door should generally be close to the stair (if you're adding one), and ideally you won't have a railing post right up against a window.
Stairway Location
When heading to your backyard, it's most convenient to walk out the door and only a short distance to the stair, so keeping the stair entry point near the door is helpful. Having the stair continue straight out or wrap a corner may provide some flexibility and make your deck fit your yard better.
It's good to know how high your deck is and how many steps the stair will have, so you can plan out where the bottom of the stair will land. Making sure it ends on your patio or center of the yard is more convenient than having it land in front of a tree, pool edge, or volleyball court. If you prefer to have the stair off to the side of the deck, then you may need to adjust the width or location of the deck to make sure your stair ends in an appropriate spot.
Furniture and Activity Space
Do you hope to use your deck to grill, eat, lounge, entertain, or all the above? On smaller decks, it's more crucial to know what furniture you hope to have on the deck, especially when it's a snug fit.
Below are a few different deck sizes with different arrangements of furniture on them: a 66" round table, a 36"x72" rectangle table, and a gas grill with an overall width of 58".
