Section 6 of Chapter 40A is hereby amended by striking the second paragraph and inserting the following new paragraph:-

A zoning ordinance or by-law shall provide that construction or operations under a building permit shall conform to any subsequent amendment of the ordinance or by-law unless the use or construction is commenced within a period of not more than twelve months after the issuance of the permit and, in cases involving construction, unless such construction is continued through to completion as continuously and expeditiously as is feasible.  Construction or operations under a special permit or site plan approval shall conform to any subsequent amendment of the zoning ordinance or by-law or of any other local land use regulations unless the use or construction is commenced within a period of two years after the issuance of the permit and, in cases involving construction, unless such construction is continued through to completion as continuously and expeditiously as is feasible.  For the purpose of the prior sentence, construction involving the redevelopment of previously disturbed land shall be deemed to have commenced upon substantial investment in site preparation or infrastructure construction, and construction of developments intended to proceed in phases shall proceed expeditiously, but not continuously, among phases.