This zoning 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 zoning 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 Board of Appeals may award a special permit pursuant to Section 10.4 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. The following types of changes to nonconforming uses may be considered by the Board of Appeals:
5.2.1. 
Change or substantial extension of the use.
5.2.2. 
Change from one nonconforming use to another, less detrimental, nonconforming use.
The Board of Appeals may award a special permit pursuant to Section 10.4 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. The following types of changes to nonconforming structures may be considered by the Board of Appeals:
5.3.1. 
Reconstructed, extended or structurally changed.
5.3.2. 
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 from the Board of Appeals; provided, however, 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 Board of Appeals.
Nonconforming single- and two-family residential structures may be reconstructed, extended, altered, or structurally changed upon a determination by the Inspector of Buildings that such proposed reconstruction, extension, alteration, or change does not increase the nonconforming nature of said structure and provided that the gross floor area of the nonconforming structure is not increased by more than 25%. For the purposes of this subsection only, the term "reconstruction" shall mean the renovation of a portion of a preexisting structure rather than demolition and replacement thereof.
The following circumstances shall not be deemed to increase the nonconforming nature of said structure:
5.5.1. 
Insufficient area. Alteration to a structure located on a lot with insufficient area which complies with all current setback, yard, building coverage, and building height requirements.
5.5.2. 
Insufficient frontage. Alteration to a structure located on a lot with insufficient frontage which complies with all current setback, yard, building coverage, and building height requirements.
5.5.3. 
Other encroachment. 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.
5.5.4. 
Exceptions do not apply. In the event that the Inspector of Buildings determines that the above exceptions do not apply, the Board of Appeals may by special permit pursuant to Section 10.4 allow such reconstruction, extension, alteration, 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, the term "reconstruction" shall not mean the total demolition and rebuilding of the premises, which is governed by Section 5.7, below.
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 the Board of Appeals, by the grant of a special permit, may authorize the reestablishment of a nonconforming use or structure where such reestablishment shall not result in substantial detriment to the neighborhood.
A nonconforming structure may be reconstructed after a catastrophe or after voluntary demolition in accordance with the following provisions. For the purposes of this subsection, the term "reconstruction" shall mean the rebuilding after catastrophe or the rebuilding after demolition of the building.
5.7.1. 
Reconstruction of said premises shall commence within two years after such catastrophe or demolition.
5.7.2. 
Building(s) may be reconstructed by right if located within the same footprint as the original nonconforming structure, and if they do not exceed the gross floor area of the original nonconforming structure.
5.7.3. 
In the event that the proposed reconstruction would (a) cause the structure to exceed the gross floor area of the original nonconforming structure or (b) cause the structure to be located other than on the original footprint, a special permit pursuant to Section 10.4 shall be required from the Board of Appeals prior to such demolition.
No nonconforming use shall, if changed to a conforming use, revert to a nonconforming use.