Maryland Code, Land Use § 22-111

(1) an extension of a lawful nonconforming use throughout all or a part of a building in which the nonconforming use lawfully exists;

(2) the restoration or reconstruction of an existing lawful nonconforming use if by fire or other calamity the use has been destroyed to the extent of not more than 75% of the reconstruction value of the building in which the lawful nonconforming use was carried on;  or

(3) an extension of an existing lawful nonconforming use on the same lot as the lot existed as a single lot under single ownership when enactment of the zoning law made the then existing use on the lot nonconforming.