There shall be a side yard on each side of a building not less than five feet (5') in width, except that an existing structure may be rebuilt, structurally altered, or extended along the same line, within the five foot (5') limit, provided the structure is no closer to the adjoining property line than was the original structure and provided it is completed within two (2) years of the time the existing structure is removed or construction commenced; provided further, that on a lot having a width of less than forty feet (40') as shown by the last recorded sale, there shall be a side yard on each side of a building of not less than three feet (3') in width, subject to the same conditions as to rebuilding, structural alteration or extension. (1997 Code §17.24.040)