The charter shall also include provisions for towns to enter or leave participation in the council of governments. Said charter shall be placed before the voters in an election. The charter shall also include a method of determining approval of any increase or decrease in the county assessment authorized in this section but such method shall include approval of such increase or decrease by the member municipalities of the council of governments in a popular vote if such increase is in excess of 102.5 per cent of the previous fiscal year assessment. The charter may also include a method of determining approval of an annual budget, including fees, grants, assessments and other revenues. The charter may provide that a council of governments shall retain any powers previously conferred upon the county and its county commissioners and shall have any additional powers authorized by this section; provided, however, that said councils shall not retain any power concerning functions transferred to the commonwealth under sections 1 or 4 of this chapter, retain power to levy a county tax, or retain powers specifically denied under this section.
Mass. Gen. Laws ch. 34B, § 20