Whenever
a non-conforming use of a building or structure, or part thereof,
has been discontinued for a period of twelve (12) consecutive months,
or for a continuous period of eighteen (18) months if the building
was originally designed and constructed for a non-residential use,
such use must not, after being discontinued or abandoned, be re-established,
and the use of the premises thereafter must be in conformity with
the regulations of the district.