Hi Peggy,
It can vary widely based on what kind of window you have now (aluminum, wood, vinyl, etcl) as well as if you have brickmould casing on the exterior etc. It can be up to 5" different. Just ... read more
In all honesty, hire a professional electrician. They will know how to install the right equipment to be sure your fan remains where it belongs and is safely wired. Good luck!