Petitions for including additional territory within an existing district may be filed with the department and the proceedings provided for in Article 5 of this chapter in the case of petitions to organize a district shall be observed in the case of petitions for such inclusion. The department shall prescribe the form for such petition, which shall be as nearly as may be in the form prescribed in Article 5 of this chapter for petitions to organize a district. When the total number of landowners in the area proposed for inclusion shall be less than twenty-five, the petition may be filed when signed by a majority of the landowners of such area and in such case no referendum need be held. In referenda upon petitions for such inclusion, all owners of land lying within the proposed additional area shall be eligible to vote.