All line extensions shall require a line extension agreement setting forth the terms and conditions under which the extension will be made, duly signed by the water-works and applicant. No charges or conditions shall be obtained from or imposed upon the applicant for the provision and extension of water service, other than those contained in the line extension agreement in compliance with the water-works tariff or as approved by the Commission. One copy of each main extension agreement shall be filed with the commission within 30 days of execution; or where such agreements may exceed ten in number in any calendar year, the utility may instead maintain a file at company offices of such agreements and file a summary of such agreements by January 31 of the following year.