After the effective date of this chapter, the nonconforming uses are declared by this chapter to be incompatible with permitted uses in the districts involved. A nonconforming use of a structure, a nonconforming use of land, or a nonconforming use of structure and land in combination shall not be extended or enlarged.

To avoid undue hardship, nothing in this chapter shall be deemed to require a change in the planned construction, or designated use of any building on which actual construction has been commenced and carried on diligently prior to the effective date of this chapter. Actual construction is hereby defined to include the placing of construction materials permanently and fastened in a permanent manner. Where excavation or demolition or removal of an existing building has been substantially begun preparatory to rebuilding, such excavation or demolition or removal shall be deemed to be actual construction, provided that work shall be carried on diligently to conclusion. All of the above provisions shall not apply unless a valid site plan has been approved by the appropriate City agency and such approval is still in effect.