Except as otherwise provided in this section, the lawful uses of land or buildings existing at the time of adoption of this chapter may be continued, even though such use does not conform to the regulations specified by this chapter for the district in which such land is located. Any lawful use existing at the time of the adoption of this chapter or any amendment thereof which, if such existing lawful use would require a special permit in the district in which it is situated if it had been newly created while this chapter was in effect, may be continued and shall be deemed to be a nonconforming use, but any modification, change or extension thereof, except as specifically permitted, shall be subject to the issuance of a special permit as provided in this chapter.