(a) The following lawful nonconforming uses of land may be continued:
(1) A use of land which existed prior to the effective date of this Zoning Ordinance.
(2) A use of land existing at the time of the annexation.
(3) A use of land existing at the time an amendment is made to the Zoning Ordinance which changes such land to a more restricted district or extends the jurisdiction of the Zoning Ordinance to the land use.
(b) The lawful use of a building located upon any land, except as provided in § 17-3102 below, may be continued although such use does not conform with the provisions of this Zoning Ordinance, and such use may be continued throughout the building if no structural alterations are made therein, except those required by law or ordinance. If no structural alterations are made in such building, a nonconforming use of the building may be changed to another nonconforming use of the same or more restricted use classification. The foregoing provisions shall also apply to any uses of buildings which may be made nonconforming by any subsequent amendment or change of this Zoning Ordinance.