17 to 22 Inches is Standard
According to columnist Tim Carter of Ask the Builder, “The window seat depth from front to back is also importantThe preferred depth should fall somewhere between 17 and 22 inches”.
It Depends on the User
The window seat’s purpose is important to consider when choosing the width. Sleek Home, a remodeling and home ideas organization, advises taking into consideration how much storage space you want to create beneath the window seat as well as whether you’ll be using it to relax in when deciding upon dimensions.
A wider window seat is generally more comfortable than a narrow one.
Use the standard range as a guide when deciding upon width, but also consider how the window seat will be used. Sleek Home notes: “If you want a window seat for napping and lounging around to read a good book then building the seat too shallow will probably make your time spent on the seat uncomfortable.
In this case, you could even extend the depth of the window seat from 22 to 24 inches”.