Modern Window Stools
For a clean look, skip the casing and install a low-profile stool.
A lot of modern designers eliminate traditional window casings in favor of the clean look that comes from simply returning the drywall to the window. While drywall is fine on the sides and top, the stool (the inside shelf of the sill) needs to be a more durable material. Here, I’m using painted MDF that overhangs the drywall by only about 1⁄2 in., and extends about 1 in. past each side of the window opening. This is a style decision, and you could easily overhang the stool farther for more of a nod to traditional design.