Notwithstanding any provision of chapter 183A to the contrary, the organization of unit owners of a condominium may petition the board of health in a city or town to enter into a betterment agreement pursuant to this section to finance the repair, replacement or upgrade of a septic system serving a unit, one or more of which is used for human habitation provided that such system comprises part of the common areas and facilities. Such agreement shall: (i) be approved by a majority of the unit owners benefited by the repair, replacement or upgrade of the septic system or any combination of such septic system improvements; (ii) include an identification of the units and unit owners subject to the agreement and the percentages, as set forth in the master deed, of the undivided interests of the respective units in the common area and facilities; and (iii) include a statement by an officer or trustee of the organization of unit owners certifying that the required number of unit owners have approved the agreement. As between the affected unit owners and the city or town, such certification shall be conclusive evidence of the authority of the organization of unit owners to enter into the agreement. A notice of such agreement shall be recorded as a betterment in the registry of deeds or registry district of the land court where the master deed is recorded and shall be otherwise subject to the provisions of chapter 80 as provided for in this section. The assessment under such agreement may be charged or assessed to the organization of units owners but shall not constitute an assessment of common expenses. Instead, the allocable share of the assessment, prorated on the basis of the percentage interests of the benefited units in the common areas and facilities, shall attach as a