Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 59, § 5O

(a) In any city or town that accepts this section, the board of selectmen or select board of the town, the town council of a municipality having a town council form of government, the city manager, with the approval of the city council, in the case of a city with a plan D or plan E form of government, or the mayor, with the approval of the city council, in all other cities, may establish a property tax exemption for real property classified as Class One, residential in the city or town. To qualify for the exemption, the property shall be: (i) rented at an affordable housing rate, as determined by the city or town and in accordance with the United States Department of Housing and Urban Development guidance and regulations; (ii) rented on a yearly basis; and (iii) occupied year-round by a person or persons whose household income does not exceed an amount to be set by the city or town; provided, however, that said income shall not be more than 200 per cent of the area median income. The property tax exemption shall be for an amount determined by the city or town; provided, however, that the amount shall not be more than the tax otherwise due on the parcel based on the full and fair assessed value multiplied by the square footage of the housing units rented and occupied by a person or persons whose household income is not more than the income limit set pursuant to clause (iii), divided by the total square footage of a structure located on the parcel. Assessment of property seeking an exemption under this section, if by an income approach to value, shall assume fair market rent for all units. The property owner seeking the exemption shall submit to the city or town any documentation the city or town deems necessary, including, but not limited to, a signed lease and proof of the occupying person or persons' household income, to confirm the eligibility of the property for the exemption under this section.

(b) A municipality may adopt ordinances or by-laws to implement this section.