It's easy to construct a window seat out of premade upper wall cabinets. The great thing about this method is the framing has already been done for you. Follow these tips to construct a window seat out of pre-made wall cabinets.
Measure the Space
Measure the length of the window and the distance between the floor and the bottom of the window casing.
Also, measure any extra area beyond the width of the window that you want to incorporate into the window seat.
If there are angles, make note of them.
Sketch out the Design
Make a detailed diagram of the seat you want as seen from above.
Note how long and how wide you want the seat, and note any angles.
At your home improvement center, look at wall cabinets. You will want the ones that are about two feet high, otherwise, the seat will be too tall.
Decide if you want to use the doors as storage or merely as an ornament, and get appropriate door hardware. You can get European hinges that are invisible from the outside and have storage that nobody can see.
Figure out which combination of cabinets will fit into your space. You won't be able to cut them down. If your space is too large for 2 cabinets but too small for 3, get 2 and plan on filling in with either corner cabinets or pieces of trim wood you can get to cover such gaps.
If you find yourself getting confused, tell one of the experts in the millworking section, and they will help you pick out what you need. Don't worry if the cabinets aren't as deep as your window seat; you'll be adding a plywood sheet that will stabilize any open areas.
You'll need a piece of ¾" plywood big enough to cover the silhouette of your window seat, with about a ½" overhang on the front and side edges. It can be in a couple of pieces, just so the seam doesn't fall along the edges of two cabinets. Sometimes you can get the plywood precut at the home center, and you may be able to get the trim pieces cut to size as well.
Get screws long enough to go diagonally through the cabinets' framing plus an inch or so, and screws long enough to go through the plywood plus about an inch.
Install the Cabinets
If your cabinets are unstained, it's easier to stain or paint them before you install them.
Measure out your space and position the cabinets where they need to be.
Drill pilot holes diagonally through the framing and into the floor (into the joists where possible), then use the screws to anchor the cabinets to the floor; this is called "toenailing." (If you don't want to drill the floor, you can skip this step but be prepared for a little sliding around during use.)
Add the doors.
Add the plywood sheet(s), anchoring them in several places to the framing of the cabinets.
You can finish off your window seat by adding decorative molding to the edges of the plywood and painting or staining to match, or by stapling fabric over the plywood.
Some people pad the plywood itself to make a cushion, but your seat will be more comfortable and versatile with separate cushions.