The final plat original tracings shall be filed with the City Planning Commission not later than twelve months after the date of approval of the preliminary plat; otherwise it will be considered void unless an extension is requested by the developer and granted in writing by the Commission. A copy of the final plat thus filed shall be transmitted to the City Engineer. If found to be in full compliance with the formal provisions of this chapter, the engineer will return such plat to the Commission, together with certification of approval.