Adding more Voxels to the mix

Hi I makeing my voxels like this.

   void Start ()
         {
             for (int a = 0; a < HightY; a++)
             {
                 for (int b = 0; b < LengthX; b++)
                 {
                     GameObject P = Instantiate(VoxelPoint, new Vector2(a,b), Quaternion.identity) as GameObject;
                     P.transform.localScale = new Vector3(1,1,1);
                 }
             }
         }

and what each voxel makes a mesh by assigning top left and topleft neighbours at the start via raycast like this. What I want to be able to do is add more voxels at runtime like this. Can anybody point me in the right direction as to how to do this all my attempts have been by instantiating more voxels along the top left bottom and right neighbours but that dosn't work long term.