The city council shall make its own determination as to whether the proposed amendment meets the standards outlined in Section 23.96.040 and may approve, modify or disapprove, the recommendation of the planning commission. Any significant modification of the amendment, not previously considered by the planning commission during its hearing, may be, but is not required to be, referred to the planning commission for report and recommendation. The planning commission shall not hold a public hearing on the proposed modification. Failure to report on the proposed modification within forty (40) days, or such longer period as the council may designate, shall be deemed an approval of the proposed modification.