The boundaries of any district may be altered, and contiguous or noncontiguous territory may be annexed thereto. The legislative body may by resolution fix a time and place for hearing upon the question of annexation of such territory to an existing district. The resolution shall describe the boundaries of the territory proposed to be annexed. The legislative body may by resolution determine that each parcel of territory annexed or to be annexed to a district should pay its proportionate share of the cost of any improvements constructed prior to such annexation. No such parcel may be annexed until the full amount of its contribution has been determined and paid. All such amounts paid to the local agency shall be placed in the special fund of the district and may be used for the purposes for which the proceeds from the annual assessments may be used. The date set for the hearing on the proposed annexation shall be at least three weeks from and after the date of the adoption of the resolution setting the hearing.