Should I install stained glass over my existing window or replace it?

There are two ways to go about it. A lot of the projects I work on, the homeowner wants the stained glass more for its decorative and character elements. In that case, I install the stained glass window right in front of the existing window. I make it to size, slide it into the frame area, and sandwich it with trim so it's secured and looks like it's part of the wall. The existing window stays in place behind it.

The other option is a full exterior-facing installation where the stained glass replaces the window entirely. That requires more attention to weatherproofing. You have to seal well around the edges to prevent water getting into the framing, and make sure the window has solid structural support to withstand the elements over time.