No such nonconforming building, if 50% or more of the assessed value thereof is destroyed due to any cause, shall be restored in nonconforming form or location or for the continuance of a nonconforming use, except that such a building, if destroyed accidentally due to fire, explosion or other cause, may, if so permitted by the Zoning Board of Appeals, be restored in substantially the same form and location but without extension of the nonconforming use.