Any lawful building, structure or use of premises existing at the time of the enactment of this chapter or any subsequent amendment thereof applying to such building, structure or use of premises may be continued although such building, structure or use of premises does not conform to the provisions thereof, except that any undeveloped lot in a subdivision which was not properly approved by the Planning Board and/or filed in the office of the County Clerk and whose area and/or depth are less than the specified minimum lot requirements and average density requirements of this chapter shall be considered in violation of this chapter.