Variances. The Board shall have the power to authorize a variance for a particular use of a parcel of land or to an existing building thereon after public hearing for which notice has been given by publication and posting as provided in Subsection F of this section and by mailing to all interested parties to grant upon appeal or upon petition with respect to particular land or structures a variance from the terms of this bylaw where the Board specifically finds that owing to circumstances relating to 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 bylaw would involve substantial hardship, financial or otherwise, to the petitioner or appellant, and the desirable relief may be granted without substantial detriment to the public good and without nullifying or substantially deviating from the intent of this bylaw. The Board may impose conditions, safeguards and limitations both of time and for 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 rights authorized by a variance are not exercised within one year of the date of grant of such variance, they shall lapse and may be re-established only after notice and a new public hearing; 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 30 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.