This bylaw shall not apply to structures or uses lawfully in existence or lawfully begun, or to a building or special permit issued before the first publication of notice of the public hearing required by MGL c. 40A, § 5, at which this bylaw, or any relevant part thereof, was adopted. Such prior, lawfully existing nonconforming uses and structures may continue, provided that no modification of the use or structure is accomplished, unless authorized hereunder.
The Zoning Board of Appeals may award a special permit to change a nonconforming use in accordance with this section only if it determines that such change or extension shall not be substantially more detrimental than the existing nonconforming use to the neighborhood.
5.2.1. 
Permissible changes. The following types of changes to nonconforming uses may be considered by the Zoning Board of Appeals:
1. 
Change or substantial extension of the use;
2. 
Change from one nonconforming use to another, less detrimental, nonconforming use.
The Zoning Board of Appeals may award a special permit to reconstruct, extend, alter, or change a nonconforming structure in accordance with this section only if it determines that such reconstruction, extension, alteration, or change shall not be substantially more detrimental than the existing nonconforming structure to the neighborhood.
5.3.1. 
Permissible changes. The following types of changes to nonconforming structures may be considered by the Zoning Board of Appeals:
1. 
Reconstructed, extended or structurally changed;
2. 
Altered to provide for a substantially different purpose or for the same purpose in a substantially different manner or to a substantially greater extent.
Except as provided in Section 5.5, below, the reconstruction, extension or structural change of a nonconforming structure in such a manner as to increase an existing nonconformity, or create a new nonconformity, shall require the issuance of a variance; provided, however, that the extension of an exterior wall at or along the same nonconforming distance within a required yard shall require the issuance of a special permit from the Zoning Board of Appeals.
Nonconforming single- and two-family residential structures may be reconstructed, extended, altered, or structurally changed upon a determination by the Building Inspector that such proposed reconstruction, extension, alteration, or change does not increase the nonconforming nature of said structure by more than 100% gross floor area.
5.5.1. 
Permissible changes. The following circumstances shall not be deemed to increase the nonconforming nature of said structure, and a building permit may be issued:
1. 
Alteration, extension or change to a structure located on a lot with insufficient area which complies with all current setback, yard, building coverage, and building height requirements;
2. 
Alteration, extension or change to a structure located on a lot with insufficient frontage which complies with all current setback, yard, building coverage, and building height requirements;
3. 
Alteration, extension or change to a structure which encroaches upon one or more required setbacks, where the alteration will otherwise comply with all current setback, yard, building coverage and building height requirements; provided, however, that the extension of the main front, side or rear wall exclusive of any bays, porches or other minor extensions at or along the same nonconforming distance within a required yard shall also be deemed not to increase the nonconforming nature of the structure so long as no new encroachment in another required yard results.
[Amended 12-5-2016 STM, Art. 15]
If the Building Inspector determines that proposed alteration, extension or change exceeds one or more of the criteria set forth above, the Zoning Board of Appeals may, by finding, allow such alteration, extension or change where it determines that the proposed modification will not be substantially more detrimental than the existing nonconforming structure to the neighborhood. For the purposes of this subsection only, the term "reconstruction" shall not include the voluntary demolition of such structure and its rebuilding. See Section 5.7.
A nonconforming use or structure which has been abandoned, or not used for a period of two years, shall lose its protected status and be subject to all of the provisions of this Zoning Bylaw; provided, however, that by special permit the Zoning Board of Appeals may reestablish a nonconforming use or structure otherwise abandoned or not used.
Any nonconforming structure may be reconstructed after a catastrophe or after demolition in accordance with the following provisions.
5.7.1. 
Procedures.
1. 
Reconstruction of said premises shall commence within two years after such catastrophe or demolition.
2. 
Buildings reconstructed as of right shall be located on the same footprint as the original nonconforming structure and shall be only as great in volume or area as the original nonconforming structure.
3. 
In the event that the proposed reconstruction would (a) cause the structure to exceed the volume or area of the original nonconforming structure or (b) cause the structure to be located other than on the original footprint, a special permit shall be required. In the case of voluntary demolition, the special permit shall be obtained from the Zoning Board of Appeals prior to such demolition.
No nonconforming use shall, if changed to a conforming use, revert to a nonconforming use.
When a prior lawful nonconforming structure is located on a lot which does not meet current dimensional requirements, such lot shall not be changed, unless the change does not result in exacerbation of an existing nonconformity or a new nonconformity.