Extension. Any nonconforming use, building or structure may be enlarged up to, but not more than, 10% of its floor area as it existed at the time of passage of this chapter. Such enlargement must conform to all other regulations of the district in which it is located and may be used only for the existing nonconforming use. Any such enlargement, with the exception of single-family residential units, is subject to § 500-21, Site plan review.