At any time not later than the time set for hearing objections to the proposed formation, any owner of real property within the proposed district may make a written objection to the formation. Such objection shall be in writing, shall contain a description of the land by lot, tract, and map number, and shall be signed by such owner. Objections shall be mailed or delivered as specified in the notice described in Section 26561. If the person whose signature appears on such objection is not shown on the assessment roll last equalized by the county as the owner of the subject real property, the written objection shall be accompanied by evidence sufficient to indicate that such person is the owner of such property. The determination by the legislative body of ownership for purposes of this section shall be final and conclusive.