A large window planter box can aesthetically improve the appearance of a large window. It doesn't matter the style of the house structure, as painting, staining or design embellishments can help the box match the house. The most important aspect is to make sure there is enough support to hold the box onto the exterior wall. It's not difficult to build such a planter box, if you have some fundamental carpentry skills and a few basic tools.
Decide on the width and length of your large flower box. Mark and cut two identical boards for the front and back. The height will depend on the width of the lumber you choose, 8, 10 or 12 inches.
Mark and cut two identical side boards for the width of your large planter box. Cut them your desired width, minus 1½ inch. This is to accommodate for the width of the front and back boards, which are each ¾ inch wide.
Put the two side boards inside the two longer boards, at the ends. This will create a frame-shape.
Screw the frame-shape together. Use your drill and screwdriver attachment to set screws along each edge of the longer boards (front and back). They will go through the long board and into the sides of the side boards. Place a screw ever 2 to 3 inches.
Place the wood frame-shape onto a piece of ¾ inch plywood. Use your pencil to draw around the frame. Cut the plywood, along the pencil marks.
Place the cut piece of plywood on top of the wood planter frame. Both sides, the frame top and bottom, should be the same measurement. Place screws through the plywood and into the top of the wood frame. Work the screws in every 2 to 3 inches.
Use a drill bit attachment to drill holes through the plywood, randomly. They will provide proper drainage for the plants, so the water does not stay sopped up at the bottom of the planter box.
Seal the large planter box with an exterior stain, paint or clear finish, inside and out. Let the large planter box dry overnight.
Find the studs in the structure of the exterior wall. Tap on the wall until you hear the difference between the hollow and the solid sound. The solid sound is the location of a stud. For large planter boxes, screw in an L-bracket at every stud, for the most possible support.
Place the window planter box onto the L-brackets. Move the box out, close to the edge of the bracket (so the bracket does not show). Screw the bracket onto the bottom of the flower planter box. Fill the box with soil and plants.