Within thirty days after the submission of a plat, the planning commission shall approve or disapprove the plat. If the plat is not approved or disapproved within that time, the plat is deemed to have been approved, and a certificate to that effect must be issued by the commission on demand. The applicant, however, may waive the requirement that the commission act within thirty days and may consent to an extension of the period. The commission shall state the grounds upon which any plat is approved or disapproved, and written findings upon which the decision is based must be included within the records of the commission. Any plat submitted to the commission must contain the name and address of an individual to whom notice of a hearing must be sent. No action may be taken by the commission upon any plat until the commission has afforded a hearing thereon. At least five days before the date fixed for the hearing, a notice of the time and place of the hearing must be sent by registered mail to the address shown on the plat. Public notice of all hearings also must be given. Every plat approved by the commission may be adopted by the commission as an amendment of or addition to the master plan without further hearing.