(3) 2x4 @ 70-1/2"
(10) 2x4 @ 25-1/2" (includes back support)
(1) 2x4 @ 12-1/2"
(6) 2x4 @ 72"
(1) 2x4 @ 70-3/4" (back support--cut to fit)
Build the seat framing using 2-3/4” screws at all joints, 2 per joint. Recommend using an exterior grade glue. Check seat framing for square by taking diagonal measurements from opposite corners and adjusting until the two diagonals are equal.
Build one arm using 2-3/4” screws. The center support on edge is flush to the inside and will be used to tie in the seat framing in next step.
Attach seat framing to arm using 2-3/4” screws. Use at least 8 screws here, more may be needed depending on the strength of your screws. This joint supports the weight of one side of the sofa.
Add legs on armless side. Make sure the legs are attached square to the seat framing. Use 2-3/4” screws.
Drill two pocket holes set for 1-1/2” thick material on one end of the back 2x4. Attach to the arm top board with 2-1/2” pocket hole screws. Then attach opposite end of back to top of single back leg with 2-3/4” screws.
Lay seat boards down on seat and screw down, keeping outside edges flush with approximately 1-1/2” gap in between boards. Screw to all seat framing boards with 2-3/4” screws.
To add the optional back supports, to the arm, first screw the remaining 25-1/2” long 2x4, inset ¼” from the inside of the arm. Use 2-3/4” screws.
Measure and cut to fit the back support board. Screw from both the arm, back leg, and top with 2-3/4” screws.