Section 2A. (a) Real property for the purpose of taxation shall include all land within the commonwealth and all buildings and other things thereon or affixed thereto, unless otherwise exempted from taxation under other provisions of law. The assessors of each city and town shall determine the fair cash valuation of such real property for the purpose of taxation on the first day of January of each year. Notwithstanding the foregoing, in any city or town which accepts the provisions of this sentence, buildings and other things erected on or affixed to land during the period beginning on January second and ending on June thirtieth of the fiscal year preceding that to which the tax relates shall be deemed part of such real property as of January first.

Mortgages upon buildings or other things, which with the land upon which they are erected or to which they are affixed are taxable as real estate defined herein, shall be deemed mortgages of real estate for the purpose of taxation, and shall be taxed under sections eleven to twelve B, inclusive.

(b) The assessors shall determine the fair cash valuation of such real property according to section thirty-eight; and if the city or town has been certified by the commissioner pursuant to subsection (c) shall classify such real property according to the following uses:—

''Class one, residential'', property used or held for human habitation containing one or more dwelling units including rooming houses with facilities designed and used for living, sleeping, cooking and eating on a non-transient basis, including a bed and breakfast home with no more than three rooms for rent. Such property includes accessory land, buildings or improvements incidental to such habitation and used exclusively by the residents of the property or their guests. Such property shall include: (i) land that is situated in a residential zone and has been subdivided into residential lots; and (ii) land used for the purpose of a manufactured housing community, as defined in section thirty-two F of chapter one hundred and forty. Such property shall not include a hotel, or motel. Such property may be exempt from taxation under other provisions of law.

''Class two, open-space'', land which is not otherwise classified and which is not taxable under the provisions of chapters sixty-one, sixty-one A or sixty-one B, or taxable under a permanent conservation restriction, and which land is not held for the production of income but is maintained in an open or natural condition and which contributes significantly to the benefit and enjoyment of the public. In a city or town that has elected to adopt section 2A of chapter 61, section 4A of chapter 61A or section 2A of chapter 61B, class two, open-space shall include land taxable under chapter 61, 61A or 61B.

''Class three, commercial'', property used or held for use for business purposes and not specifically includible in another class, including but not limited to any commercial, business, retail, trade, service, recreational, agricultural, artistic, sporting, fraternal, governmental, educational, medical or religious enterprise, for non-profit purposes. Such property may be expressly exempt from taxation under other provisions of this chapter.

''Class four, industrial'', property used or held for use for manufacturing, milling, converting, producing, processing or fabricating materials; the extraction or processing of materials unserviceable in their natural state to create commercial products or materials; the mechanical, chemical or electronic transformation of property into new products and any use that is incidental to or an integral part of such use, whether for profit or non-profit purposes; and property used or held for uses for the storage, transmitting and generating of utilities regulated by the department of public utilities or the department of telecommunications and cable. Such property may be exempt from taxation under other provisions of law.

Where real property is used or held for use for more than one purpose and such uses result in different classifications, the assessors shall allocate to each classification the percentage of the fair cash valuation of the property devoted to each use according to the guidelines promulgated by the commissioner.

Real property which is exempt from taxation under section five shall be classified according to said guidelines.

(c) Classification of real property shall not be implemented in any city or town until the commissioner has certified in writing to the assessors of such city or town that the assessments on the real property that they propose to make are at full and fair cash valuation as required by section thirty-eight and that a majority of its assessors are qualified to classify its property.