Cut seven 32” decking boards and place them upside down in the hatch frame. Screw and glue two 2x3s perpendicular to the boards. This will attach the boards to each other as well as support the lid for serving as part of the flooring.
Paint one side of the 4’ x 8’ sheet of plywood with a bright, cheerful color (the exterior side) and apply chalkboard paint to the other side (interior side). Make a 2x4 frame for the plywood. The frame should be flush with the plywood on three sides and 2 inches shorter than the plywood on the bottom where it will attach to the deck base. Screw the plywood to the 2x4s on the chalkboard-paint side.
Have someone help you raise the wall up vertically and attach it to the deck. Use 5- or 6-inch nails or wood screws, insert at least three into the top through the 2x4 frame, and three through the side where the plywood overlaps the deck. Make sure it is secure.
Measure to fit the interior dimensions of the playhouse about four feet up from the deck. Remember, you want the loft to attach to the 2x4 frame, not the plywood and shiplap. The easiest way to build the loft decking is to assemble the frame outside of the playhouse. Screw together four 2x4s in a square with one interior joist in the middle.
The ladder will use one of the angled side support beams as a side rail. To make the second rail, cut a 2x4 at the same angle as the support. Then cut the steps that will ascend to the loft.
Put at least two screws in each end of the rails to keep the steps from shifting.
The roof hatch is a 4’ x 10’ piece of white polycarbonate roofing framed with 2x4s. Unlike the deck frame, the hatch needs to stay thin, so lay the 2x4s out on their flat sides and make a notch in the two long ends so they fit around the shorter boards. Then pre-drill two holes in each joist and screw them together. Use a framing square to make sure the corners are 90 degrees, then add corner and center supports to keep it square.
Trace a rectangle on the bottom-left side of the plywood wall. Make sure it’s big enough for a child to fit through, but shorter than the 4-foot loft. Drill a starter hole to fit in the jigsaw and cut out the door. Repeat the process for a small window on the loft level.
Rip one 8-foot 2x4 lengthwise to make two 2x2 hatch legs. Pre-drill a hole in each end that is slightly larger than the width of the bolts you’re using. Attach to the bottom corners of the hatch with a bolt acting as a pivot point allowing for legs to rotate and fold in.
When the hatch is closed and the legs are folded up against the hatch frame, drill a hole through both the leg and the frame so you can to insert a stabilizing pin to keep the legs in the closed position. Then with the hatch open and the legs supporting it, drill another stabilizing hole into the deck frame for the same pin to be used when hatch is open. Attach a bracket for the leg to rest against when open and stabilized.
The bench can serve as either more storage or as a planter. Attach plywood sides with screws, measured to fit the deck opening, and make the lid by gluing three 2x4s together and then screwing trim scraps on either end to secure them. Use contrasting colors to paint the bench walls and lid.