A built-in wall bench can add extra seating to outdoor areas, under a kitchen window, on the porch or even in a bathroom if you are lucky to have one big enough to accommodate it. Wooden wall benches are fairly straightforward DIY projects and will not tax the budget or the mind with fancy equipment or complex joinery. Adjust the dimensions to suit your available space, and remember to take those dimensions with you to the lumber yard as it can save you a good amount of work if they can size the lumber there.
Form two planks of 65 inch 2-by-3 lumber and two planks of 10-inch 2-by-3 lumber into a rectangle frame, with the 10-inch planks on the inside of the 65-inch planks so that the total outer width of the rectangle is 14 inches.
Glue the joints of the wood and secure together with two countersunk 3-inch wood screws through each corner joint. Insert a third plank of 10-inch 2-by-3 lumber across the centre of the frame and secure it in place as you did the frame.
Center four planks of 70-inch 1-by-3 lumber across the frame. There should be about 2.5 inches over on each end, with the outer two planks flush with the sides of the frame and the other two evenly spaced between. Secure the bench slats to the frame with two 2-inch wood screws countersunk into the end pieces of the frame and the central support plank for each slat.
Measure up 17 inches from the floor and use a level to make a straight line, 65 inches long. Locate the wall studs on the line using a stud finder and mark their position.
Position the frame against the wall on the line and secure it in place with four 3-inch wood screws driven through the back of the frame into each stud position.
Trim the ends of three planks of 2-by-2 lumber measuring 22 inches to opposing 45-degree angles using a mitre saw or a hand saw and mitre box. Slot each plank diagonally between the two end planks of the bench frame and the central support plank, and the wall. The angled ends mean that the planks will sit flat against the underside of the bench as well as the wall.
Fix the diagonal supports in position using two countersunk 3-inch wood screws through each end of the diagonal supports into the wall and the bench underside. Finish the bench with your wood finish of choice.