Except as herein after provided, this bylaw shall not apply to structures or uses lawfully in existence or lawfully begun, or to a building permit or special permit issued before the first publication of notice of the public hearing on this bylaw or any amendments thereto, 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.

Construction or operations under a building permit or special permit shall conform to any subsequent amendments to this bylaw, 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 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.

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

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.

The following types of changes to nonconforming structures may be considered by the Zoning 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, 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.

Nonconforming single- and two-family residential structures may be 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 that such reconstruction, extension, alteration or change does not increase the gross floor area of the structure by more than 100%.

The following circumstances shall not be deemed to increase the nonconforming nature of said structure and a building permit may be issued:

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 bylaw; provided, however, that by special permit the Zoning Board of Appeals may reestablish a nonconforming use or structure otherwise abandoned or not used upon a finding that such reestablishment shall not result in substantial detriment to the neighborhood.

Any nonconforming structure, including a nonconforming single or two-family dwelling governed by § 5.5, may be reconstructed after a catastrophe or after voluntary demolition in accordance with the following provisions.

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 an increase 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.