The 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 the 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 below in Nonconforming Single- and Two-Family Residential Structures,[1] governing single- and two-family homes, the 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; the extension of an exterior wall at or along the same nonconforming distance within a required yard shall also require the issuance of a variance from the Zoning Board of Appeals.
[1]
Editor's Note: See Section 5.5.
Nonconforming single- and two-family residential structures may be extended, altered, or structurally changed upon a determination by the Building Commissioner that such proposed reconstruction, extension, alteration, or change does not increase the gross floor area of said structure by more than 100% and that one of the following circumstances shall apply, in which case the proposed extension, alteration, or change shall be deemed not to increase the nonconforming nature of said structure:
1. 
Alteration 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 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 to a structure which encroaches upon one or more required yard or setback areas, where the alteration will comply with all current setback, yard, building coverage and building height requirements.
If the Building Commissioner determines that proposed alteration, extension or change exceeds one or more of the criteria set forth above, the Zoning Board of Appeals may, by special permit, 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.
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 the bylaw.
For the purposes of this subsection only, the term "reconstruction" shall mean the voluntary demolition of such structure, or reconstruction after a catastrophe, and its rebuilding. Any nonconforming structure, other than a nonconforming single- or two-family dwelling governed by the Nonconforming Single- and Two-Family Residential Structures section,[1] 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.
[1]
Editor's Note: See Section 5.5.
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.
When a lot is changed by eminent domain so as to become deficient in area, frontage, building setback, or lot coverage, any structure located thereupon shall be considered a nonconforming structure subject to the rules of this Nonconforming Uses and Structures section.