Section 20. The following words, wherever used in this section and in sections twenty-one to twenty-three, inclusive, shall, unless a different meaning clearly appears from the context, have the following meanings:—

''Low or moderate income housing'', any housing subsidized by the federal or state government under any program to assist the construction of low or moderate income housing as defined in the applicable federal or state statute, whether built or operated by any public agency or any nonprofit or limited dividend organization.

''Uneconomic'', any condition brought about by any single factor or combination of factors to the extent that it makes it impossible for a public agency or nonprofit organization to proceed in building or operating low or moderate income housing without financial loss, or for a limited dividend organization to proceed and still realize a reasonable return in building or operating such housing within the limitations set by the subsidizing agency of government on the size or character of the development or on the amount or nature of the subsidy or on the tenants, rentals and income permissible, and without substantially changing the rent levels and units sizes proposed by the public, nonprofit or limited dividend organizations.

''Consistent with local needs'', requirements and regulations shall be considered consistent with local needs if they are reasonable in view of the regional need for low and moderate income housing considered with the number of low income persons in the city or town affected and the need to protect the health or safety of the occupants of the proposed housing or of the residents of the city or town, to promote better site and building design in relation to the surroundings, or to preserve open spaces, and if such requirements and regulations are applied as equally as possible to both subsidized and unsubsidized housing. Requirements or regulations shall be consistent with local needs when imposed by a board of zoning appeals after comprehensive hearing in a city or town where (1) low or moderate income housing exists which is in excess of ten per cent of the housing units reported in the latest federal decennial census of the city or town or on sites comprising one and one half per cent or more of the total land area zoned for residential, commercial or industrial use or (2) the application before the board would result in the commencement of construction of such housing on sites comprising more than three tenths of one per cent of such land area or ten acres, whichever is larger, in any one calendar year; provided, however, that land area owned by the United States, the commonwealth or any political subdivision thereof, or any public authority shall be excluded from the total land area referred to above when making such determination of consistency with local needs.

''Local Board'', any town or city board of survey, board of health, board of subdivision control appeals, planning board, building inspector or the officer or board having supervision of the construction of buildings or the power of enforcing municipal building laws, or city council or board of selectmen.