If it is stone you are playing Russian roulette with a 15" unsupported overhang. Stone has natural fissures and weak spots that can give way when someone leans on it or even at times, under it's own weight. It needs support!!

From a site called Fabricators Unlimited:

Overhangs for 2cm Granite that exceed 6 inches will require additional support structures

For an overhang more than 6" use a proper support system such as solid substrates, legs, brackets, corbels, or columns.
Overhangs for 3 cm Granite that exceed 10 inches will require additional support structures:

For an overhang more than 10" use a proper support system such as solid substrates, legs, brackets, corbels, or columns.

From the Marble Institute of America (the go-to reference for granite installations):

In designs where the countertop is cantilevered or overhanging the supports, the cantilever shall be limited to 6" (150 mm) for 3âÂÂ4" (20 mm) thick countertops and 10â (250 mm) for 11âÂÂ4" (30 mm) countertops, but in no case may the cantilevered portion represent more than 1/3 of the width of the countertop.

That one says the same as the first quote, just in more complicated language. Don't let these guys install without supports! If the overhang broke for lack of supports someone could be seriously injured.

Thanks

I forgot to mention that it is 3cm granite.