Mass. Gen. Laws ch. 59 § 5M
A gateway municipality, as defined in section 1 of chapter 40V, may, by vote of its legislative body, subject to the charter of the municipality, establish an exemption in an amount not less than 10 per cent and not more than 100 per cent of the incremental value of the market rate units contained in a certified housing development project within a housing development zone under chapter 40V, for a period of not less than 5 years and not more than 20 years. For the purposes of this section, market rate residential unit shall mean a market rate residential unit as defined in section 1 of chapter 40V. Such exemption shall be approved by the executive office of housing and livable communities, as established in chapter 23B. The executive office shall promulgate applicable rules and regulations to carry out this section.
Mass. Gen. Laws ch. 59, § 5M