This project takes advantage of three windows in perfect position to receive a window seat. The window sills in this home were damaged by animals. Replacing the sills gave rise to the decision to build in a window seat. The sills were replaced but the finish trim was left off -- the window seat will cover the area where the trim would go.
To begin, measure and cut 2" x 4" boards that will serve as ledger boards just below the window sills at the height of the window seat. Note that a 3/4" piece of melamine and a 3/4" trim piece will be installed on top of these ledger boards, so make sure to leave room.
Make sure the outside corners of the ledger boards are cut at the right angle to receive cross pieces running from corner to corner. The ledger boards should also be held back approximately 2 1/4 inches from the edge of the wall so that there's enough space to receive the cross pieces and the melamine from panel.
Using 16p nails, nail the ledger boards with the wider side of the 2" x 4" flat to the wall.
When the ledger boards are in, measure and cut 2" x 4s" to serve as the front cross pieces at the top and the bottom of the window seat. Make sure the top and bottom pieces are flush and plumb with each other.
Nail the top cross piece into the angled ends of the ledger boards, making sure there's enough room for the melamine panel.
Nail the bottom cross piece to the bottom boards.
Measure and cut a melamine panel for the front of the window seat, angling the edge cuts as needed. Put it in place, but only temporarily. Do not attach this panel yet.
Measure and cut the melamine for the top of the window seat, making sure the top piece extends over the front panel by 1/2 inch to 3/4 inches. Put the top melamine piece into place, and check for fit.
In order to purchase the right sized piano hinge, you should have already determined the size and shape (e.g. square, rectangle) of the storage access door.
Remove the top of the seat. Measure carefully and cut the opening very carefully with a circular saw (Image 1); finish with a finish hand saw. These cuts will be seen in the top of the seat, so be sure to tape the edge of the cut line before you cut to avoid chipping the melamine.
Turn over the newly cut melamine top piece and the storage access piece, and put them together again. Attach the piano hinge to the lift-up portion of the seat as well as to the underside of the window seat (Image 2). Turn the panel over and test for operability and fit.
Use an iron to adhere melamine trim to the exposed areas of the melamine.
With all the panels in place, measure along the top seat panel to add the 3/4" round trim where the top seat panel meets the wall. With caulk, the trim should cover any gaps between the top piece and the wall.
Cut the 3/4" round trim, nail the trim to the wall and add caulk.
Add baseboard or trim along the front panel to match the rest of the room.