Repairs and alterations may be made to a non-conforming building; provided that no structural alterations or extensions shall be made, except those required by law or code, unless the building is changed to a conforming use; provided, further, that where the building is non-conforming as to situation on the lot only, and does conform as to use and type of structure, in that the usage made of the building conforms to the requirements of this Chapter and the structure is of the type allowed in the district where such building may be situated, but the building is located nearer to a lot line than the minimum setback required by this Chapter, an extension may be made to such building on a side other than that which is nearer to a lot line than the required setback, so long as such extension, when completed, will not violate this Chapter as to usage or type of structure allowed in the district, or as to setback lines on the sides which were conforming as to setback prior to the extension; provided, further, that the Board of Adjustment, by special permit in the case of evident hardship, may grant an extension of a non-conforming use not exceeding twenty-five percent (25%) of the ground area of the building.