N.H. Rev. Stat. Ann. § 482:32

No other dam shall be erected to the injury of any proposed dam or any proposed increase in any existing dam which, in either case, has been found to be for the public good by the department pursuant to this subdivision, if the petitioner shall, within 3 years after such finding or within such further time as the department for good cause may grant, commence the construction and prosecute said construction with reasonable diligence.