Expanding your deck and have some handy friends? This 12 x 12-foot addition is a surprisingly doable DIY.

Small Backyard deck

A new home usually calls for a refresh by painting a few rooms and updating some fixtures. But with this house, the new owners needed an outdoor makeover right away for a very practical reason: a small deck completely enclosed the back door, cutting it off from the backyard and garage.

Backyard deck expansion

Using the existing structure as the cue for design and installation of a 12 x 12-foot addition, the plan was deliberately simple to allow for changes as the owners lived with the property. Here’s how we did it:

men setting deck foundation

PLACE THE POSTS. Dig 18-inch-deep holes for tall posts in four corners, between corner posts, at sides of stairway opening and for short posts through the center. Set leveled posts in holes with mixed concrete.

men placing cross beams for deck foundation

FRAME THE ADDITION. Screw framing 2 x 4s to outer posts. Using joist hangers, attach floor joist 2 x 4s to frame with screws, spacing joists no more than 2 feet apart. Attach a center support beam under joists.

deck expasion floor

INSTALL THE FLOOR AND RAILING. Attach 12-foot 2 x 6s to each floor joist with deck screws. You can use 1×6 boards if you prefer but we chose the larger boards to match the existing deck. Leave a space about the thickness of a yardstick between each board to allow for drainage. Attach railing and spindles to posts.

deck expansion stairs

CALCULATE YOUR STEPS. Establish your stairway landing with a solid surface such as rock, concrete or railroad ties. Measure from landing to deck surface and divide by 7¾ inches to get the number of risers needed (round up to next whole number).

stairs from the deck

CREATE A STAIRWAY. Place a pair of stringers 3 feet apart and attach to deck frame with stringer hardware. Center 2 1 x 4 treads over each riser and secure with screws. Set newel posts for stair railing. Attach top and bottom rails and spindles.

man sealing deck expansion

GIVE IT A FINISH. Wrap the bottom of the deck in privacy lattice to create a backdrop for the garden and to prevent critters from taking up residence. Attach lattice to deck posts below the deck surface with screws and mask joins with trim. Coat the lattice, deck and railing with outdoor stain (first remove any old stain on existing deck with deck wash and let dry at least 24 hours).

finished Deck expansion

Lumber and supplies plus a lot of elbow grease came together in a surprisingly roomy and inviting addition to the small deck. Here’s what it cost:

  • Treated lumber $370
  • Concrete mix $18
  • Hardware $85
  • Stair components $80
  • Privacy lattice $100
  • Stain $60
  • Pathway pavers and rock $70

TOTAL $783

Don’t miss our next post to see how we planted a mixed garden in the retaining wall below the deck. And with end-of-season plants, too!


© Caruth Studio

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