Another alternative to adding permanently to the quilt is perhaps to just layer it ? I recently made a quilt for my mothers birthday only to find out that she had under estimated the height of her mattress ( we live several states apart ) so the measurements I used when making the quilt ended up being about 10 inches too short ( 5 inches per side ). What she did was buy a flat sheet in a coordinating color and she places that on the bed before the quilt on top, so that the sheet hangs to cover the gap, and with the coordinating colors it looks very nice. She said she actually prefers that instead of having me try and enlarge the quilt, since now she can use the quilt in the spare room ( with a shorter mattress ) when she wants to swap out her own bed , without it being too long for the spare bedroom !