To authorize upon appeal, or upon petition in cases where a particular dimensional variance is sought for which no permit is required, with respect to a particular parcel of land or to an existing building thereon, a variance from the terms of this chapter where, owing to conditions especially affecting such parcel or building but not affecting generally the zoning district in which it is located, a literal enforcement of the provisions of this chapter would involve substantial hardship, financial or otherwise, to the applicant, and where desirable relief may be granted without substantial detriment to the public good and without nullifying or substantially derogating from the intent or purpose of this chapter, but not otherwise. In exercising these powers, the Board may impose limitations of time, and a continuation of the dimensional variance permitted may be conditioned upon compliance.