Conversion of a single-family detached dwelling or a building devoted partly to single-family dwellings and partly to commercial use into a two-family dwelling, provided that the lot area per family shall be at least 3,000 square feet, the lot area per commercial use shall be at least 3,000 square feet, at least two off-street parking spaces shall be provided per dwelling unit, and parking shall be provided for the commercial use in accordance with § 27-609, Subsection 21, of this chapter.