Section 10. The permit granting authority shall have the power after public hearing for which notice has been given by publication and posting as provided in section eleven and by mailing to all parties in interest to grant upon appeal or upon petition with respect to particular land or structures a variance from the terms of the applicable zoning ordinance or by-law where such permit granting authority specifically finds that owing to circumstances relating to the soil conditions, shape, or topography of such land or structures and especially affecting such land or structures but not affecting generally the zoning district in which it is located, a literal enforcement of the provisions of the ordinance or by-law would involve substantial hardship, financial or otherwise, to the petitioner or appellant, and that desirable relief may be granted without substantial detriment to the public good and without nullifying or substantially derogating from the intent or purpose of such ordinance or by-law. Except where local ordinances or by-laws shall expressly permit variances for use, no variance may authorize a use or activity not otherwise permitted in the district in which the land or structure is located; provided however, that such variances properly granted prior to January first, nineteen hundred and seventy-six but limited in time, may be extended on the same terms and conditions that were in effect for such variance upon said effective date.

The permit granting authority may impose conditions, safeguards and limitations both of time and of use, including the continued existence of any particular structures but excluding any condition, safeguards or limitation based upon the continued ownership of the land or structures to which the variance pertains by the applicant, petitioner or any owner.

If the rights authorized by a variance are not exercised within one year of the date of grant of such variance such rights shall lapse; provided, however, that the permit granting authority in its discretion and upon written application by the grantee of such rights may extend the time for exercise of such rights for a period not to exceed six months; and provided, further, that the application for such extension is filed with such permit granting authority prior to the expiration of such one year period. If the permit granting authority does not grant such extension within thirty days of the date of application therefor, and upon the expiration of the original one year period, such rights may be reestablished only after notice and a new hearing pursuant to the provisions of this section.