This Zoning Ordinance 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 chapter, or any relevant part thereof, was adopted, but shall apply to any change or substantial extension of such use, to a building permit or special permit issued after the first notice or said public hearing, to any reconstruction, extension or structural change of such structure and to any alteration of a structure begun after the first notice of said public hearing to provide for its use in a substantially different purpose or for the same purpose in a substantially different manner or to a substantially greater extent except where alteration, reconstruction, extension or a structural change to a single- or two-family residential structure does not increase the nonconforming nature of said structure. Such prior, lawfully existing nonconforming uses and structures may continue, provided that no modification of the use or structure is accomplished, unless authorized hereunder.

Construction or operations under a building permit or special permit shall conform to any subsequent amendments to this chapter unless the use or construction is commenced within a period of not more than 12 months after the issuance of the permit and in any case involving construction, unless such construction is continued through to completion as continuously and expeditiously as is reasonable.

The Board of Appeals may grant 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.

The following types of changes to nonconforming uses may be considered by the Board of Appeals:

The Board of Appeals may grant 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.

The following types of changes to nonconforming structures may be considered by the Board of Appeals:

Except as provided in § 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, including the extension of an exterior wall at or along the same nonconforming distance within a required yard, shall require the issuance of a variance 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 Building Commissioner that such proposed reconstruction, extension, alteration, or change does not increase the nonconforming nature of said structure. The following circumstances shall not be deemed to increase the nonconforming nature of said structure as long as such addition does not increase the habitable floor area of the original structure by more than 100%.

Alteration to a structure located on a lot with insufficient area which complies with all current setback, yard, building coverage, and building height requirements.

Alteration to a structure located on a lot with insufficient frontage which complies with all current setback, yard, building coverage, and building height requirements.

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.

In the event that the Building Commissioner determines that the nonconforming nature of such structure would be increased by the proposed reconstruction, extension, alteration, or change, the Board of Appeals may, by special permit, 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.

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 by-law; provided, however, that such use or structure may be restored to its protected status by special permit from the City Council.

Any nonconforming structure may be reconstructed after a catastrophe or after demolition in accordance with the following provisions:

Reconstruction of said premises shall commence within two years after such catastrophe or demolition.

Building(s) as reconstructed shall be located on the same footprint as the original nonconforming structure, and shall be only as great in gross floor area as the original nonconforming structure.

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 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.

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 section 5.0.