A trailer or commercial coach which is used full or part time as a sales office shall not be placed on a site as described in section 8.20.170 of this article until a building permit has been issued for one or more residential units of the subdivision. Such trailer or commercial coach shall be removed when eighty percent (80%) of the dwelling units have been sold or by the time six (6) months has lapsed after the final inspection by the city of the last residential unit in the subdivision. (Prior code § 66404)