What's a bay window without a window seat? An unfinished and underutilized space waiting to be transformed! Building a window seat is a basic "build-in" project and here's how you can do it.
Step 1: Plan It Out
Since bay windows are not all created equal, you'll need to measure the angles of the walls. The standard is 135° but there are other variations and the "as built" angles will probably be different. Walls are never perfectly straight, corners are never perfectly square...you get the picture. In the end, you don't have to be super exact since a piece of trim will cover the gap between the wall and the seat, but the angle needs to follow the wall.
How deep you'd like your seat to be is the next question. I build this one to be 24" deep so we could use a rug runner instead of cushions. Runners are 21"-23" wide. How deep your seat is will also determine how wide it will be. At 24" deep, mine turned out to be 8' 5" long. Since sheets of plywood are 8' long it would have been easier to make the front shorter than 8' long and let the depth work itself out. In that way you could rip an 8' piece of plywood for the front whereas I used 2 pieces.
On www.askthebuilder.com I found a sketch of the framing for a window seat (pic 2). The height they recommend is 19" with a 21" deep seat. You can see from the drawing that a window seat is actually a short floating wall achored to 2x4 cleats on the wall. The front can be wood, sheetrock or other materials since it is only decorative. The one thing I did differently from the drawing was to place the 2x4s on edge as opposed to typical wall framing. This makes a sufficiently strong wall and also creates a bit more storage space inside the bench.