Petitions for including additional territory within an existing district may be filed with the commission. The proceedings provided for in this part in the case of petitions to organize a district must be observed in the case of petitions for inclusion. The commission shall prescribe the form for such petitions, which must be substantially the same as the form for petitions to organize a district. If the number of landowners in the area proposed for inclusion is less than twenty-five (25), a referendum is not required if the petition is signed by a majority of the landowners of the area. In referenda upon petitions for inclusion, all landowners within the proposed additional area are eligible to vote.