In new bathroom, custom vanity is inset 24 inches into 60 inch wide allcove. Granite with 10" backsplash. Designer calls for frameless mirror, full width, and from ceiling to overhang backsplash by 4 inches. Overhang is accomplished by horizontal 2by4 's behind mirror secured to studs and mirror somehow secured to these studs. Any suggestions.

 

Drywall over the studs and use mirror adhesive. Put a block under the mirror until the adhesive is set.

Better pray that your opening is square.

 

Agree with the added drywall layer, reduce the 2x4's to 1x4's if needed.
The mirror adhesive should be applied in 3/8" diameter beads running the full length of the mirror. Beads should be 5 to 6 inches apart.
You need a full wall, not just strips, especially since you have no mechanical supports.

 

Mirror is offset from wall. I have a dry walled wall currently, so attach done 1by4,s to existing wall horizontal, then attach drywall to those, then glue mirror to drywall.

What about using 1/2 ply instead of drywall and gluing mirror to that. The plywood could be screwed through the 1by4 and into studs.

 

Personally, I don't get the designers vision with the overhang or the covering up of all that granite. You also now have a vulnerable edge of a very large mirror that I see chipping out during the course of normal use. Once you glue it up, there is little chance of getting it off the wall ever again. Is the mirror custom cut to clear the granite on the 10" side splashes as well? Will you be hiding a light inside the overhang to give the counter and stone a glow?

And X mentioned it, but worth emphasizing, use mirror adhesive only - other adhesives will react with the silver in the mirror and bleed through.