To grant, upon appeal or petition, with respect to particular land or structures or to an existing building thereon, a variance from the terms of this bylaw where, owing to circumstances relating to the soil conditions, shape or topography of such land or structures and owing to conditions especially affecting such parcel or such building but not affecting generally the zoning district in which it is located, a literal enforcement of the provisions of this bylaw would involve substantial hardship, financial or otherwise, to the appellant, 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 bylaw. The Board of Appeals shall not grant use variances.