A nonconforming structure may not be altered or expanded, except by variance. A nonconforming structure may be altered, reconstructed, externally, or structurally modified, provided that such alterations, reconstruction, extension, or structural modification does not make any portion or portions of the existing structure more nonconforming. A nonconforming structure cannot be reconstructed after demolition, except when the structure was demolished by an act of God, fire or flood or is found unsafe as determined by the Building Inspector or his/her designee. A nonconforming building or a building occupied by a nonconforming use may be strengthened and made safe.