Nothing herein contained shall require any change in the overall layout, plans, construction, size or designated uses of any development, building, structure or part thereof, for which the official approvals and required building permits have been granted before the enactment of the chapter, the construction of which, conforming with such plans, shall have been started prior to the effective date of the chapter and the completion thereof carried on in a normal manner within the subsequent six-month period, and not discontinued until completion, except for reasons beyond the builder’s control.