Each such space shall be clearly defined or delineated. Each mobile home space shall have an area or lot size of not less than 10,000 square feet, with no boundary line thereof to be less than 75 feet in length; provided, however, that mobile home parks which, at the time of the adoption of this chapter, existed lawfully with mobile home spaces that do not comply with the foregoing area requirements may continue to operate and shall be excused from such compliance, but the area of such mobile home parks shall not be increased or extended unless such additional area shall conform to all of the requirements of this chapter.