Before approval of the roll a notice shall be published once a week for two consecutive weeks in a newspaper of general circulation in the improvement district, stating that the roll is on file and open to inspection in the office of the secretary, and fixing the time, not less than fifteen or more than thirty days from the date of the first publication of the notice, within which protests must be filed with the secretary against any assessments shown thereon, and fixing a time when a hearing will be held by the commissioners on the protests. Notice shall also be given by mailing, at least fifteen days before the hearing, a similar notice to the owners or reputed owners of the land in the improvement district as they appear on the books of the treasurer of the county in which the real property is located. At the hearing, or any adjournment thereof, the commissioners may correct, change, or modify the roll, or any part thereof, or set aside the roll and order a new assessment, and may then by resolution approve it. If an assessment is raised a new notice similar to the first shall be given, after which final approval of the roll may be made. When property has been entered originally upon the roll and the assessment thereon is not raised, no objection thereto shall be considered by the commissioners or by any court on appeal unless the objection is made in writing at, or prior to, the date fixed for the original hearing upon the roll.